I would say even earlier - say mid March. April 15 would be too late if you already have another offer on the table, since you have until April 15 to accept funded offers.
Unfortunately some programs are pretty bad at this. There was a thread on here last year where an applicant got his rejection for the Fall in August and he replied with an angry email. They're like that date that tells you they're gonna call you, but they don't. This is rare though, and you will most likely hear back from them (hopefully positive news). Good luck!
Well in some situations Master's programs can be harder to get into than PhD programs. Look at Minnesota's statistics for Computer Science - http://www.grad.umn.edu/data/stats/ad/1019600.html#apc they tend to admit a higher percentage to PhD than to MS. That's probably because they tend to get many more Master's applicants that PhD applicants. This may be different for other fields though, since I'd imagine that Computer Science gets a lot of MS applicants who want to go into industry afterwards.
Refreshing your technical skills is good, and it's always a plus to be able to put on your resume that you're good at Matlab, but I think it's more important to have a solid knowledge of the current research taking place in your field or sub-field. So if you already have a good idea of what you want to do, I would suggest reading recent papers in that area, so you know what the cutting edge is when you start.
Congratulations on your excellent choices! Does any of the two come with funding? If one did and the other didn't, then that could have a big impact on your choice. I'm not too sure about the two programs you're deciding between, but I would certainly imagine that LA has a significantly higher cost of living than Pittsburgh. Are the Master's programs you got admitted to terminal, or research-based? If they are terminal (or course work based) then that could limit your possibility of getting into a PhD program afterwards. If you want to do a PhD, then a research based MS is much better than a terminal one. Good luck!