Have a read of the other topics in this forum ("John Lyon vs MTS/HABS", "Good state school v indy", "St Albans Boys or Parmiter's?") - just substitute the name of your schools and most of the info is relevant to your question.
We are confused whether to go for the indies or WGBS. Over a long run, we can somehow manage indies fees but still trying to understand if spending £15K on fees is really worth.
My personal opinion is that the indies are not worth £15K over a school like WGBS. If it was a choice between an indie and a badly performing school or money was not a factor, then I'd say yes, but in your case, probably not.
Can one accept place at indy by paying the deposit and also accept the Grammar School at the same time? This will buy time to make an informed decision (hopefully!). I know the risk here is losing one term's fees if we eventually go with the Grammar option. But is this possible?
Yes. It's not really fair and probably frowned upon, but if you need the time to make the right decision, then that's your choice. As lara said though, it would be the right thing to do to let the other school know as soon as you've made a decision.