1.Yes, indeed my existing version of TextMate will work just fine, however it won't get any new features/updates in the long run since probably most resources will be spent developing TextMate 2. I will be using an abandoned product.

2. My TextMate license is only a few months old, so probably there will be the felling of buyer's remorse.

3. Since MacVim is free if there will ever be a totally new MacVim version I can easily switch to it with no additional costs involved.

So in the short run, yes sticking to TextMate is far easier, but in the long run I feel like switching to MacVim will be a better investment.