I must admit...

i play both, and i think both have it own merit, you should play both if you can

SPAZ doesn't appeal to me. Not my kinda game. I have FTL though. It's alright... it's everything that I expected, but I guess it's not my kinda game either, so I suppose that fits. However, it's very good for what it is.

That said, if you find out FTL is on sale, Google for the author's site. He tends to match a lot of sales. So if it's half off or something, you can get it on the author's site for the same. The author will give you a Steam key plus a DRM-free copy (it's through the Humble system, and if you have a Humble account you can add it to your library there). Very cool!

Also, consider buying the soundtrack, but I didn't. There was an AMA on Reddit and the guy who did the soundtrack seemed cool. A lot of people vouched for him as well. I only didn't buy because I'm not interested in video game music, mostly. A lot of people are, and if you're one of those people, well, your money could do worse.

SPAZ's engine is good but the gameplay is a bit too grindy near the end of the game unless you plan to die many times trying to accomplish near impossible missions with your slow/no firepower/weak shields ships.