Krispykreme: I can understand the situation you are in, because i was once considering both too.
Both are very good shops, but i must say both are on different platform. Meaning, they are strong in their own individual style and design.
Just sharing my person opinion here..( may not be agreed by others of course) I went to Wedding Present first, and i liked their gown design, majority of the gown are made with duchess satin, with embroidery... have very sweet design and very grand design. However i was told by a frend that it is important which designer you use there, cos there is good and not so good one.
As for Silh designs, they are those with lots of lace, and tulle design, slightly lesser duchess satin. i noticed this difference, and thus paid a little more attention to their details. Then i realised that their details and beadings that they use are very selected. They may not have a variety of beads / sequins, but they do use carefully selected beads that suit the lace or gown. End up, the gown looks so much more attractive. eg: their tulle gowns, i've seen a few that has nothing but just sequins, it still looks opulent. I chose Silh eventually because i feel more comfortable wearing silh gowns, and i've not heard any bad comments about any of their designer.
So end day, it really depends the kind of design that you like, the details that you prefer, and your comfortability level with the shop / designer. Dont be shy to try their gowns, you may discover more details, your likes and dislikes on your own.
GOOD LUCK!