Ok, so making a few assumptions:
You're using different processes or transfer modes for root and cap operations.
You don't want to have to post weld dress the root pass on the pipe interior, or use backing.
You intend to RT the welds to verify defect condition,
You have high volumes of welds to make in various diameters.
You want to do this robotically if possible.
You're not afraid to pay for top of the line equipment.
You have careful controls in place to ensure acuurate fit-ups and stretegic tacking operations.

I recommend a Miller Axcess-E 450.
This would allow the ability for using RMD tranfer for the root pass, with accu-pulse and/or spray transfers for fill and cap.
With all the additional options for fine tuning every nuance of the weld, this can offer ease of use reliability for acceptable welds while maximizing efficiency.
The Centerpoint functions that the Axcess-E gives will also provide weld monitoring and a digital signature for each weld, which can be used for quality acceptance and documentation.

You could also use Lincoln Powerwave with STT and Checkpoint technology, and Fronius TransPuls with CMT and Explorer technology with the same result.
Each have options for both manual and robotic capability.

In this case, each manufacturer would offer an effective solution with competitive pricing, it just comes down to who can provide the best service in your area.

Tim