I hadn't been reading around too much lately, just saw this thread, so I'll give you what info I can as an owner of both ^_^ Mine are all 60some cm dolls though, so this information is mostly only relevant to 1/3rd size dolls. There are different factors if you're talking about smaller ones.
As Zirconmermaid said, what you want to do with your doll will probably be the biggest determining factor as to which is best for you. Also you're own visual preferences.
First, most have mentioned the main points, but Iplehouse dolls aren't exactly typical of most resin bjds out there. Overall, I think silicone bodies can't be beat for photography since the skeletons pose and stick well, there are no joints and all the ones I've seen so far are well sculpted with realistic anatomy. I really love bjds, but I hate joints. However, your choices for silicone dolls are very limited right now as there are not many makers. To me, Iplehouse has the most beautiful, realistic and anatomically correct face and body sculpts in the bjd world, but there's definitely a big tradeoff with the posing abilities. And the fact that things stick to silicone is a unique factor as well :/ powdering your doll helps a lot, but you have to do that occasionally as upkeep. It's not as big a deal unless you have a lot of fur from cats or dogs in your house. I do have a dog and she has no limits so it does bug me when fur gets to my doll when I'm handling her.
Durability:Iplehouse, like most bjds are resin. It's a kind of plastic, not particularly fragile, but especially with large dolls you really don't want to let them fall over or drop from any height as pieces can shatter or break, especially fingers. If you get a scratch on them you can sand it out, although if that happens on a painted area you may have to paint and finish that part of the doll again. As far as I know, you can pose them without limit and not cause any damage.
Silicone dolls, well, dropping isn't an issue obviously hehe. Small scratches or wantnot can be fixed with Silpoxy and then putting plastic wrap over that so that it will cure smooth and not be particularly obvious. Tears can be fixed similarly, but depending on the severity the repair won't be so pretty and that area will be weaker even after being fixed. I've only had my girl for 4 or 5 months, so not that long really, but I think I fear the silicone tearing and imagine her being more fragile than she actually is. I dressed and posed her a bit without incident. You definitely want to store the doll on it's back without any parts folded or creased, because if you leave silicone creased too long, when you unfold it, the silicone can crack and tear, and if you have her standing, the skeleton will eventually poke through the feet. Silicone does not age well from what I hear so it becomes more fragile as it gets older.
All in all, a silicone doll is likely to have a shorter shelf life unless you shelter it.
Posability:For big Iplehouse dolls, this is where they differ most from other bjds. Their elbow and knee joints are not peanut or ball shaped like other bjds, they have a cylindrical shape with prongs on the end that the 2nd limb slides along. I don't know why they did this, because it's actually really bad for posing. It's hard to get the joints out of the socket to do double joint posing and then if a limb slips off the rails it's hard to put it back on if the doll is wearing clothes. Sometimes you have to take the clothes off to slide the limb back in place. They also have really bad ankles which is what I hate most about them. I mean I love all 4 of my Iplehouse dolls, but ug those ankles... There is no ball joint for them, there's just a divide between the foot and the leg and as you're handling the doll, the foot often slips toe up and/or out to the sides, so half the time, whenever you move the doll, you're constantly having to put the feet back in place before you put the doll back down again. The walls of the sculpts are also very thick and heavy so even tightly strung they can't hold all poses simply due to the weight of their limbs. There are photography tricks you can use to get around that though.
Silicone: They pose VERY easily except for the fact that they can't stand unassisted as the silicone is quite heavy and can't really be balanced to stand alone. The skeleton is constructed so that the poses stick and stay until you move them. If your doll is in a kneeling pose, you can pick it up, spin it around, throw it in the air and the limbs won't move unless you purposely rearrange them. The only thing is that if the skeleton breaks, it can't be fixed. There is no way to take apart the doll to fix anything that breaks and put it back together again. As far as I've handled my doll though the skeleton doesn't really seem to be particularly fragile except for the fingers, because they are wires which will fatigue after being bent so many times. The rest of the skeleton has hinges which are more durable.
Customizability:Iplehouse: Same as all resin bjds, highly customizable. You can sand or dremel things off, or use epoxy to add on and there pastels, acrylics or other water based materials can be used on them.
Silicone: You can put a resin head on a silicone body, but there aren't many silicone heads available right now and you cannot customize the shape of silicone. Most silicone heads available right now are in anime style, but there are a few that are a bit more realistic. You can't trim anything or add anything on. Paint-wise, the only thing that sticks to silicone is silicone, so silicone paints are the only thing that you can use and they can be a bit pricey. You don't need much if you're painting just a doll though, so a sample kit is more than enough, but it's harder to paint silicone than it is to paint resin. It doesnt' blend easily by brush, so it's best to airbrush and airbrushes aren't the easiest things to use. If you're not proficient with an airbrush, it's best to pay for the doll to come with body blushing and faceup. I think they all come with faceups by default, but blushing is extra.
If you want to do some extra reading besides what I put here, this is the thread from when I first got my Iplehouse dolls:
viewtopic.php?f=8&t=6454And here's 2 threads about my silicone girl:
viewtopic.php?f=8&t=6463&start=20viewtopic.php?f=8&t=7174
Posted: Mon Dec 31, 2012 2:49 am
In the end, all dolls have their pros and cons. It seems like everyone here is already listing the goods and bads of silicone vs. ball joints, so it's not like you're going in 'unarmed' either.