As always it depends on your needs. If you want it portable to lug around, and the amp isn't some monster that consumes several hundred mA, there's no need to go overboard with a > 500mA wart. For example, a wart I use with a Pimeta (consumes a little under 80mA) is capable of about 200mA and entire wart with regulator board inside measures 3L x 2W x 1.75H" (excluding cord and plug).
On the other hand, if you're not interested in portability so much and have no plan to dedicate it permanently to a particular amp(s), look at how your Tread is configured, what it's max. current capability is. For example, if it has LM317 that's 1.5A max, BUT if you aren't going to put a big heatsink on the regulator, the realistic max is a lot lower. If your Tread can't handle 500mA, there's not much point to a wart capable of more than that, except if it'll be operating in a higher ambient temp or a static current drain at reasonably high % of the transformer rating, then the resultant heat could make it worthwhile to either choose a larger capacity transformer or drill some vent holes in the wart casing... which if you don't know the internal layout of the wart, can be risky (trying to do it blind instead of cracking open the wart).
Personally, I prefer warts that have screw-shut cases instead of welded or glued shells. That tends to make them slightly larger though.
As for specific warts, no need to be picky about brand, keeping your technical criteria in mind, and you may find there is a cost savings in adding it to some other parts order because shipping cost for a wart alone can easily be as much or more than the wart cost.