I took the top-down approach, and my first sale was to a pro-paying market. Credits do not matter at all in short fiction, only in novels (and in SF&F, often not even then.)
I recommend the top-down approach if SFWA membership is important to you, obviously. Their list of qualifying markets is a good place to start, not for money, but for prestige and possible eventual membership. Seriously though guys, "top dollar" for a short story isn't even going to pay a month's rent, so don't even consider money. If a magazine is respected and is SFWA-eligible, it's "top," even if someone else pays more.
Here's the list:
http://www.sfwa.org/join-us/sfwa-membership-requirements/#shortfiction
But there are plenty of non-SFWA markets that are also very well respected. Some are not eligible simply because they're too new, like
Lightspeed, others don't pay enough to qualify but you should still thank your lucky stars if you get in, such as
Shimmer. (I only give one example of each so people don't get offended they're left out

)
I honestly don't see any reason for submitting to "lesser" markets first unless you are genuinely desperate for a quick turnaround time and not just impatient (for example, your
Campbell clock is ticking and you just need to get a certain amount of fiction out there where people can read it, STAT.) But even that advice only works if you know which "lesser" markets have a really quick turnaround time.
Strangely, the fastest turnarounds (at least, for rejections) seem to be some of the top markets.
The Magazine of Fantasy & Science Fiction, Clarkesworld, Fantasy Magazine, and Lightspeed all stand out as having less-than-a-month response times, so there is really no reason not to submit to those.
Hope this helps.
