grunzster:
So are you saying air is good enough if I'm not diving Trimix?
OR...
It's not even worth the cost and trouble of having the second tank for the dry suit unless I'm diving Trimix?
Well, kinda both and a bit more.
The entire reason to have the second tank for the suit, and to take the hit on the harness (yes, its not much, but its not zero either!) is because He has extremely high thermal conductivity, and if you inflate your suit with that, you will get a surprise!
Once you've already got the separate tank, if the Argon is cheap for the amount you need, you may as well use it, although I'm not so sure that it offers much improvement over air in the "side tank" even if you ARE diving Trimix.
When you look at this scientifically, the difference is about 10-15% maximum in terms of thermal conductivity. That's not zero, but its also not a lot.
Now if you are NOT diving Trimix, then why take the extra complexity? There's no good reason for it, IMHO. Further, you need TWO tanks of Argon really, one big one on the boat to purge your suit with, and the second to inflate with. If you're not going to purge the suit before you go underwater, you may as well not bother as the concentration won't be high enough to matter, and if you try to purge with a 6cf tank you'll get a second surprise (its not enough gas, really, for that.)
If you ARE diving Trimix, then its trickier. You already need the bottle, reg, etc. You may as well fill with Argon if its reasonably-priced where you got the Trimix. But if the Argon is expensive where you are, then you might try filling that inflation bottle with air and see if you can tell the difference and if its worth the extra coin.