Sorry I didn't see this earlier - I don't get any sort of notification when someone posts here (here's a better place to post).
First off - Both Jalapeno and SalsaPress are free and don't (I believe) break each other. So no reason not to use both!
Second - Jalapeno is built (I believe) to work with Salsa's actions - allowing you to embed information about Actions and Actions into Pages / Posts. SalsaPress is mostly geared towards Sign Up Forms and Events - allowing you to embed embed the forms in widgets and posts.
Third - I think Jalapeno is no longer being actively supported (and I've heard from some folks may not be working currently). I try to support SalsaPress off and on - as well as add new features (post Nov 2012). Here's the list of things I'd like to do with it.
Let me know if this helps!