What are pros/cons for 1TB 5400 rpm hard drive vs 512 GB SSD? I don't know what either of these things are.

An SSD is a solid state device, like a USB flash drive A 5400 rpm hard drive is a conventional drive that uses a spinning disk:

http://www.pcmag.com/article2/0,2817,2404258,00.asp

Here are typical benchmarks on the OWC site:

http://eshop.macsales.com/Reviews/Framework.cfm?page=/Benchmarks/CS5BenchmarkPag e-MacBookPro17.html

So in general , twice as fast.

I always buy my stuff from OWC. Never a problem with compatibility like other sources.

Generally speaking, this is a no-brainer. SSD is much, much better: faster, no moving parts, less power consumption, etc. I would get the SSD unless you really need 1 TB storage.