the pod hd and the m13 share the same effects... so any attention to one would likely show up on the other....
having said that I don't think that the effects have been changed significantly in the pod from the m13
As Zap noted, there really isn't any difference between the M13 effects and the HD series effects. The only effects I can think of that the HD500 has that the M13 doesn't are the Vocoder and the Mic Preamp. And since both of those are really designed around using a vocal mic, they wouldn't necessarily translate very well to the M13. Whether or not Line 6 will update the M-series any further, I don't know. Personally, I kind of doubt it, but I could be wrong.
I guess I kind of take the conservative approach when it comes to things. If it ain't broke don't fix it. There are a lot of M5s, M9s and M13s on people's pedalboards at the moment. To me they're more comparable to stompboxes than they are all-in-one amp modeling products. Even though they overlap a lot with the HD stuff, they're really aimed at different types of players.