I've used both also. I used an Edge at a company I used to work almost 10 years ago, and I just bought a new Summa DC4sx about 4 months ago. I always liked the Edge, but what made up my mind was the fact that I only need to buy one machine, and I could print/cut straight out of Corel. It has been a while since I used the Edge, so I don't know much different they are now but from what I read, it didn't sound like much had changed. I would have had to purchase the Edge, the plotter and the pricey software that I never cared much for anyway. I haven't had any problems with contour cuts, I've done larger vehicle graphics and smaller 1" decals without problems. The only problems I've had, I've been able to make a call to Summa support and have it figured out within 15 minutes. You are more limited on materials and supplies, but I have a friend with a 54" ink jet that I sub out work too. In a perfect world, I'd have the room to have one of each, a thermal and an ink jet! 
One thing that does irk me a little about my Summa is that on every job, it runs almost a foot of material before the print/cut starts, so I'm wasting a lot vinyl. I try to group jobs together and print everything all at once to cut down on that. I don't remember the Edge doing that. If you you're going to do a lot of small quantities of small decals, the Edge might be better for you. If you're going to do larger stuff, you might want one of the Summa's. One more thing to keep in mind, since to you don't have to change out the cartridges in the Summa, it's a little easier to multi-task. You send it to print and leave the room to do something else and it'll be done when you're ready for it.
One thing that does irk me a little about my Summa is that on every job, it runs almost a foot of material before the print/cut starts, so I'm wasting a lot vinyl. I try to group jobs together and print everything all at once to cut down on that. I don't remember the Edge doing that. If you you're going to do a lot of small quantities of small decals, the Edge might be better for you. If you're going to do larger stuff, you might want one of the Summa's. One more thing to keep in mind, since to you don't have to change out the cartridges in the Summa, it's a little easier to multi-task. You send it to print and leave the room to do something else and it'll be done when you're ready for it.