Thanks for the detailed replies.
The J 29 I can actually get for about 300 dollars more than a 15 at a local store, due to it having been taken back a number of times-but as far as I can see is in a near mint condition. I used a Hummingbird for a short period but the last guitar I had was an old J 45, which was a family instrument that I really liked but felt I should quit hogging and hand over to the next generation.
I don't know when or if I'll ever be able to afford a model 'above' the 29, but I wanted to get something a bit like the 45 and the Hummingbird, that would be made well enough to be kept in the family for a while. That's the priority I guess, and I'm still a little worried that the 15 might not be in it for the 'long-haul. But the 300 extra is a lot for me.