I've got an Odyysey clarinet and thinking of upgrading to either the Buffet E11 or one of the second hand Howarth clarinets. My clarinet is in Bb, but I know there are A's and Eb's. As I already have a Bb, would it be a much wiser decision to get an Eb or an A?
I can play the Bb one with no problems, would there be much of a difference between the others in terms of music theory and actual fingering on the clarinet itself.

I would get a little better Bb clarinet like an R13 before you think about getting an Eb or an A. If youre going to need to double on one of those than youre going to need a good Bb first and foremost. And I would get an A over an Eb because its a little more practical. There's more opportunities to be playing an A over an Eb.

well, definitely go with upgrading Bb before you look into Eb and A- those are doubling instruments, and htere aren't always parts. A's are used in a lot of orchestra pieces, but often (in newer releases) there are transposed parts for Bb so it's not crucial to have an A unless you're really involved in an orchestra or something. Eb's have occasional parts in orchestra music and often in band music, but they aren't always a crucial part- often it's doubled or cued in other instruments. one of my friends plays Eb but since we have a small clarinet section, most of the time its more useful for her to stay on Bb and so she only plays important/prominent/solo parts on the Eb.

difference in playing-wise, the A is more similar to the Bb, and the Eb is a little different, being substantially smaller. i think you have to be more careful to control the higher end of the range.

Eb's are really cute though

Thank you very much for your help. I was considering an Eb, but definately would now only consider a much better Bb upgrade.
Thanks for your help.