I can't answer this, honestly, because I NEVER judge on feedback percentages alone.
Either one of those sellers could be wonderful, or a complete nightmare.
The one with 20+ feedbacks might just be someone, genuinely, cleaning out their designer-filled closet and all of the 98.9% positive seller's negs could be retaliatory ones, possibly from bad sellers, as well as buyers and/or for silly things, like realistically high p&p [for good packaging and safe shipping methods], or the seller putting the correct value on customs forms).
Conversely, the 20+ one may be the new ID of a previously NARUed scammer and the all the negs of the 98.9% seller might be for sending fakes, nothing at all and/or SNAD items.
It's impossible to say, without taking ALL factors into consideration.
For the 600+ fb seller, I'd use Toolhaus, or Goofbay, to check out the reasons behind his negs and neutrals and for the 20+ seller, I'd want to know how long they had been a member and I'd probably contact them, if it hadn't been that long, to get a feel for what type of person they seemed to be.
In fact, assuming that neither seller seemed like a scammer (and so, you weren't already put off), I'd always advocate pre-sale contact, in any case, personally.
Sorry for the long answer to a short question, but very little in life is as simple as it may, at first, appear, unfortunately.