with straight co2 there is a possibility of carbon infusion at higher welding voltages. at out of position work your voltage is lower so it "shouldn't" be a problem but in the flat at voltages above 27 volts you are creating a low alloy steel and it will be harder than the base/parent metal and asking for cracks in the haz/weld zone. the job should have a wps and it would state what gas is allowed. for all around 75/25 is probably best
esab 7100 ultra is an excellent wire.
for structural work the wps might indicate a h8 or h4 wire and these do not seem to do so well with straight co2 but there are low hyd wires developed specifically for straight co2
http://news.thomasnet.com/fullstory/490508 i have never knowingly used them so cannot speak to their weldability, i have used so many wires i couldn't even begin to keep track of them all
what you'll have to do is weigh out the extra gas costs with the mixture gas with inexpensive low hyd wire and the cost of the upgraded wire with straight co2.
there is definitely some one within this forum that has already done these type of calculations. and of course which you like welding with
darren