definately try and find ones that protect your eyes from sunlight coming from the sides and below the frames. a lot of aviation-oriented sunnies (ie raybans) are designed like this. really good feature when flying above a layer of cloud in daytime.
i found it hard getting my latest pair of sunnies (even though it was 2 for $35, but all sunnies sold in Aus meet certain minimum 'safe' requirements) to sufficiently cover my eyes from the annoying light from below/side of the frame.
really the only ones that weren't aimed at aviation that stopped all the light coming from around the outside of the frames was those stupid bloody blowfly type ones you see all the uni fellas wearing trying to be in the lastest fasions and that look like EXACTLY the same ones as the women wear....
i had a really good pair that fitted me brilliantly for years. had them since before i began flying but unfortunately they broke. i then came to realise after having another pair (one of the 2 for $35's) broken in flight that whatever you buy, be prepared for them to break and you having to fork out for another pair, no matter how careful you are.
in short, buy what blocks light from the outside of the frames from creeping in, that fits you comfortably under a headset and which you can afford to replace easily!
as for which brand to reccommend, i cannot help sorry... shop around!