In reply to SJD: I'm assuming this is for a Conville course then! I'd probably go for a 16-19cm screw (medium-ish length) because that'll hold a static load fine during crevasse rescue and it'll also be more useful than a 22cm screw in Scotland IMHO. The major advantage of the longer screw would be in constructing Abalakov threads for abseil although I'm not sure Conville courses cover this. Do the course organisers not give you any advice regarding gear?
As for the brand, I use a mix of BD Turbos and DMM Revloutions - there's not a lot in it to be honest apart from the fact that the BDs bite a bit better to start with. I think Needlesports are doing Mountain Technology screws for £25 each at the moment, might be worth a look...
Hope this helps.