I think they can be marginally easier to get in to - not sure that's really true. It's just there seems to be a total frenzy about Tiffin, particularly as it's the most accessible from West London (whereas the Sutton ones aren't, except with a very convoluted/long journey).
I think you may find that Wilson's (one of the Sutton grammars) consistently outperforms Tiffin (Boys) in league tables, although for the girls grammars Tiffin does usually edge it over Sutton competitors.
The Kingston/Sutton grammars are super-selectives in a way that the Bucks ones aren't (I don't think). Check any league tables and you will find that the SW London borough ones year on year generally outperform the Bucks grammars by some margin.
You need to look at the entry requirements for all, TBQH.
It is possible to get into a super-selective with additional input from Year 5, BUT probably becoming less and less easy to do so. The demand for places is ramping up every year. Most children seem to start being tutored from Year 3/4 (and many continue to be so right through secondary school too, even once they're in grammar schools).
And grammar schools don't necessarily serve all their pupils to reach their potential in a way that you'd necessarily consider they would. If you've got a reasonably bright child you may find a very good comprehensive will serve them better. IMHO (having been to one myself and having a DS who is at one), grammar schools generally require DC to be self-motivated learners - they are not spoon-fed. If you've got a child who's not, you may find that they don't actually achieve as well as they should do/would do in a more 'pushy' comprehensive educational environment.
Good luck!