I've used both UltraDNS and DNSMadeEasy, although it has been years since I used UltraDNS. UltraDNS's service was great from a technical point of view - and I'm sure it has only gotten better - but we parted ways over a billing error of their making. (I'm sure they've got this issue sorted out by now, otherwise they'd be out of business.)
As you mention, UltraDNS has GeoDNS - although it's on DNSMadeEasy's road map (http://www.dnsmadeeasy.com/technology/roadmap.html), as well - and I suspect their anycast network may be larger than DNSMadeEasy's, but I don't know that for certain. I can't speak to how either would handle a DoS attack, but I suspect they're both quite familiar with mitigating them.
DNSMadeEasy comes in at a substantially lower price point, as you've noticed, and I've had no complaints about their service in the five or so years I've used them. The few times I've needed to contact support, their replies have been prompt and helpful.
Ultimately, I expect the difference in price reflects the market each is targeting. You probably won't go wrong choosing either - so why not choose both (or another pair of reputable DNS providers)? If you can make two providers work together to provide DNS for your site, you remove your DNS provider as a single point of failure.