Toyota over Nissan any day. This is especially true since you are looking used. However, I would be wary of Carmax. While they say they have the best deals, they are often limited and you might find yourself buying a Truck you never get to see, NOT A GOOD IDEA. While Carmax has been great for alot of people, they're still a dealer and still worried about pushing vehicles through inventory as quickly as possible.
I was in the same situation as you. Wanted a truck, not a car, and wanted something with power, but not a guzzler. Well, I was underwhelmed by the power of 4cyl and 6cyl trucks, not great for hauling and in the 4cyl you must avoid any situations which require quick acceleration (i.e. short on ramps, getting past slow cars on the highway, etc.). I actually ended up with a V-8 Ford F-150. Great truck, terrible gas milage (duh). Anyway, long story short, ended up trading the truck in for a V-6 Honda Accord Coupe due to the fact that I was not using the truck AS a truck very often (i.e. hauling junk, carting larger payloads around, etc.).
Morals of the Story: Don't limit yourself to Carmax. Check out autotrader.com to find cars/trucks close to you that you can actually test drive and possibly take to a mechanic to check for problems (highly recommended). Beware the 4cyl truck if looking for acceleration/hauling power. Toyota over Nissan. Finally, to be the "devil's advocate", if you're really worried about gas milage, get a 4cyl car, not a truck.
Best of luck, hope you find what you want!
p.s. I'm not hating on 4cyl trucks, these are just my own personal opinions. Alot of people have enjoyed 4cyl trucks. If you still want one, I believe robav8r's suggestion is one of the best 4cyl trucks out there, but hard to find...