A ROM can be better (or worse) for battery life, depending on what you want and which ROM you chose. I've run SlimRom (4.3 based), CM aka Cyanogenmod (4.2, 4.3, and now 4.4 based). I've also tried Android Open Kang Project aka AOKP. Each ROM has pros and cons. Some offer more customization as far as the UI and also add extra features.Others ROMs have been stripped down to the bare minimum to allow for better battery life -- but I find these to be less stable (prone to crashes/reboots) and not nearly as interesting to use day-to-day.
If you're new to flashing ROMs there are a number of how-to instructional that will explain it better then I can. I will give you a basic run-down though, so you have an idea what you're getting into.
Download your ROM of choice to your computer. It should be a .zip file (I always keep different ROMs on my hard drive). Also download the correct GAPPS file. This will also be a .zip file. The creator of the ROM should provide a link to the correct GAPPS.
Connect your phone to your PC and copy the .zip to the internal storage on your phone. It's also a good idea at this point to copy any files (music, movies, documents, etc) to your hard drive for safe keeping, because you will be wiping your data and you will loose it if you don't.
Reboot to recovery
Make a NAND. Fancy name for a backup. I generally don't but I don't mind doing a setup on my phone. I should though, and it would have saved my a$$ a number of times. So do it, it will save you later!
Do a full wipe. This includes Data, Cache, and Dalvik Cache.
While still in recovery, flash/install your ROM, and the GAPPS.
Wipe Cache and Dalvik Cache again.
Reboot.
Everyone will tell you and I'll tell you too.... let the device fully boot up. It can take up to 10 minutes sometimes, so be patient. From here you set up the phone just like you did when you first got it.
Personally I really like CM 11, but if you're new to flashing ROMs I'd stay with CM 10 because they are a more stable build with fewer bugs. I also really like Action Launcher, altough I also really like Apex Pro launcher.
From now on, the world is your oyster. Keep a "clean" NAND on your computer at all times just in case things go bad -- and sooner or later they will!. By a clean NAND I mean factory stock but rooted (no ROM installed). If you have that, and WUGS you can pretty much get your phone working no matter how bad you think it's screwed up.
OK enough rambling. If I'm wrong on a point or 2 I'm sure someone will come along and correct me. The biggest thing is to read lots and do lots of research. Flashing ROMs on a Nexus is simple once you get the hang of it. At my worst I'd flash 3 different ROMS a week, or more!
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