There's a lot more to the decision to consider. What are your main welding conditions? What are your intentions for use (light gauge, heavy metal, clean or rusty steel...)? Do you have the amperage available to run heavy metal?
With the 140 you're going to pretty much be limited to gauge metal if you run it a lot with Innershield or GMAW. If you use Dual shield you can get to heavier metal and better welds as it penetrates even more than GMAW. If you pre-heat the metal to 150+ you can go even heavier.
As far as which gas to use, CO2 gives the most bang for the buck but some say the spatter is worse. It also is better at covering the weld in light breezy conditions than mix. I get about 3-4 spools of .045 Ultracore C71 / CO2 bottle compared to 1+ of .045 Ultracore M71 85/15% mix. YRMV.
There's a ton of variables between gasses and wire not to mention each individual welder, both man and machine.