Silicone, because I prefer vibrators and I like the give that silicone has, whereas glass is stiff and not always comfortable. Also glass vibrators are WAY too noisy and rattly.
I do like glass though, I think it can be fun if you get the right toy and it's very easy to clean, compatible with all lubes, and there's less drag than there is with silicone, and you can heat them up or cool them down for temperature play. I usually only use my glass toys anally, though.
I think it really depends what you want to get out of the toy. Both are great, just in different ways.