You might need to figure out what your nitrogen guy's regular route timing is. If he's got all his customers running an 18 week or longer tank, you obviously wouldn't want to buy a 16 week tank. They often know of a used tank on their route that is for sale as well.

There's probably more of the 660 straw MVE XC20 signature tanks going up and down the road every day than anything else. They're easy to handle for moving them in and out of trucks. My only complaint is if you're constantly working out of one and moving it around a lot you're going to test that 20 week hold time. If your regular route is an 18 week fill and you're not over using and abusing it, this obviously wouldn't be an issue. sementanks.com has them on sale right now. That's also a good website to explore your options.   

If you're looking for a big tank, the MVE 43/28 tanks are very good tanks. I have a pair of them and both of them will last longer than advertised. They're a big neck tank with lots of room to work, but they're not much fun to move around. The longer hold time gives you plenty of extra days in case something goes wrong or if you're storing several embryos as nearly everyone else on the route will be throwing a fit before you even get low.