I am in the market for buying a "new" instrument and I've been researching some new ideas. I've been looking for King 2B's and/or 3B/F and came across the combination of a 3B bell section with a 2B+ slide combination. I love small trombones and small bore tenors (I play on a .485 & 7.25") but I enjoy the usage of a trigger for all-around playing.

In your varying opinions, would it be better to buy the two sections separately and combine them, or would it be a better investment to just buy only 1 of the 2 different trombones?

I'd start with a cheap 3B/F. If you really want it to be smaller, you can do mods later. I think it'll be great as is though.

How about a Benge 170 alongside a 3B/F?

I have a Benge 170 slide, also .500“, which fits and works ok on my 3B/F bell section. Having said that, I rarely play that combination as I tend to prefer bigger setups. I personally prefer the reverse with the King slide on the Benge bell section or the 3B/F with its original slide. That‘s probably more about my tastes as a player, I think someone who enjoys playing .500“ might get on better.

The Benge, itself also a great Horn, could be found a bit cheaper than the small number of used 2B+ I‘ve seen.

I‘d agree with Burgerbob, try a 3B/F, if you feel later you really want a .500“ slide there are at least a couple of options to get one.