Of those two, personally I would say Zen on the basis that you're paying more for less so it should be better, IYSWIM, but in reality I'd say toss a coin and see how you get on as they are both monthly contracts and both have no problem giving a MAC code if you aren't happy.
You might want to have a look at Newnet as well, I've not used them but they look like good value and seem to have a decent reputation.
cheers,
Paul
--
paul <at> spamcop.net