For ~6 gallon batches (into fermenter) I use 25L sanke kegs (6.6 gallon capacity). I have two of this size.
For ~6.5-7 gallon batches (also into fermenter) I use the tall pony kegs I have (7.75 gallon capacity). I have two tall and one short pony kegs right now. Hope to score another tall pony shortly.
When I start making 10 gallon batches, I have a 50L keg (13.3 gallon capacity) that should do the trick. Or I'll just split it into two of the other kegs I have.
For aging batches, I have four 1/6 bbl kegs (5.16 gallon capacity) plus one 5 gallon corny keg.
I started off with the typical double bucket kit, and quickly grew to hate the bucket fermenter. I started using the Better Bottle carboys but only until I started getting sanke kegs. Now I use sanke kegs to ferment all my beers, big and small. I will use the glass carboys for mead and/or hard ciders and such, but I make those infrequently at best. I'm also rarely moving them more than about 8-10 feet at a time, so breaking them isn't a concern (same floor).
I really like using sanke kegs since there's absolutely zero light, or air, penetration into them. You cannot get that with either buckets, or carboys, without either covering them or putting them into a light tight room/closet. Not an option for me.
IMO, if you have the chance to grab a sanke keg that's a size that would work as a fermenter for you, do it. The spear/valve isn't that difficult to remove, and a soak with PBW/oxyfree will get pretty much anything out (couple with a keg/carboy cleaner on a drill and you're golden). You can use either the large orange carboy caps on the keg, get the sanke fermenter kit (machined SS) or even cram a large universal bung into it (not the one for the better bottles, the other one).
Besides, I've never heard of anyone dropping a sanke keg and having it break. Sure, it can dent, but that's about it. When it's not under pressure (such as for dispensing beer) there's zero chance of having it break. You also don't need to worry about a hot sanitizer soak doing anything to it. Hell, you can even boil the things if you ever get an infected batch.