Flexwatt is fine or just a UTH or heat cable it really doen't matter much. Personally I'd not bother with flexwatt for a small tub. You would need a small bit (6" of 3" wide) and then to wire it and such it just isn't worth it for a small single tub. You will be upgrading the set up in 6 or 8 months so I world go uth the small ones and use it for warming rats... it is only 12 $ so not really a problem just grow the pad until you hit the larger tubs.

I like herpstats myself. The others are also have good reputations too I have used a helix and herpstats. Rich is great to deal with (VE200) so no issues there either basically similar units and features, it doesn't really matter much pick one.