Matco arguably has the best battery powered impact wrench. It is shorter than most so it gets into tighter spaces and it uses a physically smaller and lighter battery (16V) so you need several batteries if you are using it all day, but it seems to be the favorite of the pros. BTW, this little monster produces 1600 ft/lbs of torque. But it is $$$$.
If you plan on getting other tools like a drill and maybe a sawsall or a circular saw, you want to pick a line that will have all the tools you will need. Dewalt, Ryobi, Makita and Milwaukee have pretty extensive lines. Chose the price point based on you expected usage. I have Ryobi, which is the lowest price point of the bunch but I do not use my tools professionally. I use them a lot but if I needed them for my job, I’d pick one of the others.
BTW, Milwaukee has an impact wrench with 1300 ft/lbs of loosening torque, Ryobi has one that is 1100 ft/lbs loosening. Mikita has a pretty good one to and I believe that Dewalt has also upgraded theirs from the one @Tester linked to.
Now, about the battery. There is some misinformation out there on this. A bigger battery is heavier, but they not only last longer between charges, they do provide marginally more power. Actually that can be significant between the smallest battery and the largest. But a newer generation battery that is smaller can proved as much or more power that a previous generation larger battery.
For example, Ryobi has 18v batteries 1.5 AH, 2 AH, 3AH, 4 AH, 6H and 9AH. A second generation 3AH would actually out perform a first gen 4AH. It has to do with internal impedance (resistance). The lower the internal impedance, the more current flow it can provide dead short. This means that the battery with lower internal impedance drops less voltage internally so it delivers slightly more voltage (and current) to the tool. The 3rd gen batteries are out and they are impressive, so much so that I think they dropped the 9AH. The third gen 4AH is lighter than the 3AH second gen.
BTW, there is no difference between a Lithium Ion 18 volt battery and a Lithium Ion 20 volt battery. They both have a no load voltage of around 21.5 volts. The actual voltage delivered to the tool depends on the internal impedance of the battery and the impedance of the tool. The 18 or 20 volt designations are just the nominal voltage under load, but that varies greatly by the tool, and to a lesser extent, the battery.