I have had both at one time or another. Those are the only two I’m aware of. There a few other sites which are non-public, but they’re by invitation only, not simply charging a fee.
Brainburst: I’ve never been a huge fan. They hype the hell out of their premium writers, but they’re pretty underwhelming. Two categories here: Players no-one’s heard of presenting semi-tested decks (Jarrod Bright, Remy Lupira), or pros presenting tech that’s one or two steps behind their current testing (Kai Budde, Jeroen Remie). I didn’t renew because, well, I can get it elsewhere for free.
Starcity: Long time fan, and they only keep growing. They have the best writers, period. Mike Flores, Stephen Menendian, Zvi Moshowitz, Philip Stanton, Osyp Lebedowicz, Chad Ellis, Antonino De Rosa, etc. All of these are essential to the format they write about, not to mention Flores’ contributions to Magic theory. Plus, they’ve published me, so that’s got to count for something.
I’ve played since 1995, and am at the PTQ, GPT level. At least, I was before my wife and I had a baby. Now, I play mostly Magic Online when I have the chance. Still, I contribute to my local groups playtesting and brainstorming sessions, and I read SCG, mtg.com, and misetings on a daily basis. I’m a frequent contributor to the message board at misetings, and I used to be on the newsgroups before they died.
If you only do one, do Starcity.