if you can't test em before purchase, get the THD, as it's most well known and therefore easier to trade on ebay. You could also get one from
www.thomann.de who offer 30 days return policy.
Weber Mass is risky as you'd have to take care of all the shipping stress and couldn't return it - and selling on ebay might be difficult due to the small number of people who know it.
Ideal case you could compare 2 or 3 power brakes in a shop with your amp and guitar ...