Hi Kristal,
Outlook is one of the most un-optimized programs from a battery perspective, that was the reason they didn't put on it RT at the beginning, with SP1 it got better but it still impacts battery life but it is still the best Business Class Email Client with all of the PIM functionality, Lync Integration and OneNote Integration I can't live without it.
All of my mail accounts are either Exchange or IMAP based so everything just syncs across devices and mail clients, so if I answer an email in one it will still show up in the other. I use the Mail App for a causal client and also to populate notifications on the Lock Screen.