In that price range, the iRoast is the hands-down winner because you can control the roast profile. In fact, you can control it a lot more than you can with lots of more expensive machines. I've used mine for over a year without complaints. (It has an attachment for a dryer hose, by the way -- so I just vent it out the range hood. Without that, the coffee smoke would permeate the house, and trust me, that's a lot worse than it sounds. A LOT worse.)
Outdoors in the winter is a bit of a problem, don't forget. Sucking cold air into a hot roasting chamber really messes with your roast times. There's plenty of good reading on coffeegeek.com about many of these issues.
The popcorn popper is apparently surprisingly good. I've never tried it myself so I can't say. The main issue, I think, is collecting the chaff.
The iRoasts have an expected life of a couple of years with regular use, not a lot more, and sometimes less (keep the warranty!!). They generate a ton of heat for such a small unit. When mine dies I might trade up for a Behmor, for a few reasons.
1. Quieter. The iRoast is a leather-lunged little devil. That's actually not a purely aesthetic problem; combined with the thick glass and small (150g) volume of beans, it means that first crack can be hard to hear with some beans. That's not insurmountable, but it's a pain.
2. Roasts more -- larger capacity.
3. People who have used both say that drum roasters make better coffee, by a hair. I haven't, but I'm curious.
4. Double-filtered ventilation system = no need to vent to outside.
5. I'm pretty much dead certain I'll be roasting my own coffee myself for a long time. Nothing at all against the local roasters (we're actually luckier than most people realize in that department!), but, the specialty coffees I want, delivered to my house, roasted to the exact degree I want, and never more than 48 hours old when I grind 'em? Yup, I'm all over that.
The big downsides would be the need to "coast" to the desired degree of roast (rather than just hit stop, as the iRoast lets you do) and the decreased control over the roast profile.
All told, though: if you're curious about it, I'd try the popper method first, especially if you can do it outdoors and have a whole summer ahead of you. It's the smallest investment with the biggest potential payoff. Then when fall comes you can think about whether it's something you want to keep doing.
Oh -- and +1 on cold brewing. The concentrate from this morning was good enough that I'm going to grind up the rest of my Mexican Feminina and set it to soaking overnight.