My Fluval Edge is by far my favorite tank I own. That being said, is is without a doubt the most difficult.
The small opening is just half of the problem. The other half is the lighting. Due to the tank configuration it is practically impossible to throw light to all corners of the tank (even with the newer led version). You'll find most people that successfully make it into a high-tech planted tank eventually use another light in front of the opening, which essentially negates the cool look of the tank.
I ended up embracing the light/dark areas of the tank instead of fighting them. I swapped out the existing lights with a few Cree LED's on a dimmer. Demanding plants get a central spot and the outlying areas get scape and plants that prefer less light. It actually can work out nice, but in the end it is work. I love it because to me it's half aquarium, half art :-)
With all that said I'd get the biggest nano you can get (as others have said). A 10g is now considered a "nano" by many, and you can do little things like de-rim the top to make it unique. Hard to beat a cool looking tank for $10 and a half hour of time. Doing the same with a 5g is awesome too.