Which horn should I use to practice?

[Brlin]

I'm a returner trombone player, played in high school and now back into it after ~27 years. I have a Bach 42 0.457 bore and a yamaha 200AD student horn 0.500 bore. I'm not sure if I'll be doing more jazz or classical playing down the line, just trying to practice and get back in the swing of things.

Which horn should I use to practice just for the sake of improvement (scales, arpeggios, articulation, flexibility exercises, tonguing, etc?) I've read that playing on larger bore horns will help you improve on small bores as well, but I also find it easier to practice the high range on the small bore.

[Doug Elliott]

Both.
I pretty much only play .500 bore for my gigs, but when I practice it's on my large bore. So I manage to stay in shape on both. I also play an Eb tuba some, and even just a few minutes on it occasionally helps keep everything else in shape.

Playing both small and large bore is a lot easier when you use the same mouthpiece rim size on both.

[Brlin]

Thank you for the suggestion! Would it make sense to alternate horns each day then? Small bore today, large bore tomorrow?

I've got a yamaha 48 on the small bore and denis wick 5AL on the bach. That seems close enough for now in terms of rim size...

Both.
I pretty much only play .500 bore for my gigs, but when I practice it's on my large bore. So I manage to stay in shape on both. I also play an Eb tuba some, and even just a few minutes on it occasionally helps keep everything else in shape.

Playing both small and large bore is a lot easier when you use the same mouthpiece rim size on both.

[AtomicClock]

I'd stick to a single horn for the first month or two. Which one? Whichever makes you want to practice more.

[Bassbone11]

^unironically this. leverage whatever you're excited about to develop some sort of practice habit and you can shape what you want from there. right now, its just about time on the face

[hyperbolica]

There will be times when you need to practice a certain horn to get ready for a particular gig, but if you can, I'd practice both horns every day as much as possible. Maybe play Real Book on the small horn and Rochut on the big horn.

I used to have trouble shifting from large to small bore, but I started using Doug's mouthpieces and now all my tenor mouthpieces use the same rim size. I have a different rim for bass bone, I can't make it work on the same rim as tenor.

If you check my profile, I've got a few different horns just because I like the different effects of different horns. I do most of my playing on bass and medium bore (0.525"), but I like to play my smaller bore (500/508) too just to keep used to it. If you play a lot of large bore and then move to small, you'll notice how much less air is required for the small bore. It's not just the embouchure that is different.