I have used both zen and magento (and messed with many others), so I think I can give some good comparisons of the two.
Zen is a great cart, it has been around for a while so it has a ton of mods. I dont know what you mean about the admin panel, but I agree that they are a little deep. To me, Zen is pretty easy to skin and modify. Zen's forums are great too, absolutely great. The downside (for me) was dependent attributes. Yes, I can get them to work, but it was WAY to time consuming.
Magento on the other hand has a lot less mods, but doesn't need a whole lot either. A lot of the features "every" cart needs are out of the box. Dependent attributes are a lot easier to function. Scriptalicous and jquery are built in - so its pretty easy to add a lot to it (though that is not my direct expertise). IMO, the magento forums are not as friendly as zen. There are a lot of unanswered questions, but magento is a cart more for developers than end users. The biggest drawback of magento is speed - it is slow. No cheap hosting accounts here. Simple Helix claims to have the fastest php in America (I have an account with them, among others) and another company (I forget the name) claims to have the fastest magento speeds in America. I would at least recommend a vps for magento, it is heavy on php and js.
The biggest advantage to magento is the multi store feature. That is awesome. Nothing else comes close. But I think it does take a little longer to learn than zen. The way everything is setup does not always make sense. There a bunch of different files with the same name, just in different locations, which can be a little confusing. It is harder to skin, but again, it does need a whole lot to look good. The next best thing about magento is that I have never once had to modify db tables, I like that.
I dumped zen a while for magento, but I still peek in on the forums to see what is new.