Hey Jon!
First of all, your going to get more than one opinion on your question because, well - we need more details regarding the material you're welding, the filler, the metallurgical properties you're looking to achieve in the finished weld, and Heat Affected Zone, whether or not you're welding the pipe from a fixed position or out of position, whether or not you're using consumable inserts or not, etc.
Niekie has a point about GMAW spray transfer (even though he wrote GTAW, hmmm - must be that fat fingered virus again!!!).
However, GMAW-P can work well provided that optimal welding parameters are met. So can a good all position FCAW wire for that matter.
In re-reading your question again, GTAW is a process whereby constant current is used, and GMAW spray is performed with a constant voltage power source. therefore, as Niekie stated in his reply, heat input is based on both electrical energy, and the welding speed.
Now RonG gave a good suggestion about using pulse GTAW to significantly reduce your heat input and in fact has been used for quite some time now on some X-ray quality pipe welds.
The other reply's suggestions made sense also.
I'm sure that you'll find an opinion that will help you decide in choosing the best process for your application... Good luck!!!
Respectfully,
SSBN727 Run Silent... Run Deep!!!