The little 6mms are definitely the sweet spot of small enough to be cheap and pleasant and enjoy, but kill with just a little more authority than a .223, a bit further out, and with much better hit probability. Ludicrous selection of good bullets available.
I like the GT but dasher would equally be fine.
The only downsides seem to be
- To maintain best performance, regular and effective bore cleaning seems more likely to be necessary than something like a .223, 6.5CM or .308
- Cost per round isn't actually that much less than a 6.5CM in my part of the world and I can't shoot NRL hunter with my 6mm hunting rifle but I could with 6.5CM