The M2 6m "HO Loop" does seem to outperform everything else, for real.http://www.m2inc.com/index2.html
I went "roving" in my van for the January SS and "roving" in tandem with me was W6YLZ. We both used identical equipment (same rigs, same transverters, same everything) and on 6m used FT-857Ds at 100W PEP for our mobile rover setups. He had the M2 HO Loop installed 2' above the roof of his SUV and I had a KB6KQ Loop about 3' above the roof of my van. They came out to be almost identical heights above ground, and every time we operated we were parked next to each other at various locations.
His square HO Loop consistently outperformed my KB6KQ round loop, every time, on every contact. I suspected something wrong with my loop, except it was making lots of contacts and its VSWR was nearly 1:1, the coax was new (and very short) and there's really nothing to adjust. The M2 just worked better.
The PAR suggestion was probably a good one.