We have been in a similar situation and couldn't decide.
I would say to look at other things like :
Tiffin is very much a city school, is your son sporty ? as far as I know they shuttle the kids to their sports grounds which are not onsite. they are strong on dance and rowing.
Wilson has beautiful grounds but not much going on in the area. As opposed to tiffin which is in a very lively environment with lots of school around tiffin girls or KGS.
what about the curriculum ?
Some grammars school aren't great on the classics/latin or Modern Foreign languages, as they focus on science and maths. is that important to you or not ? or what is important ?
Wilson is very strict and prescriptive, will that suit your son ?
look at the specific threads on mumsnet, I have learned a lot !
Good luck ! you have all done well.