Hello Phillip,
I'm still new with the GPX, going on a year now with mine but luckily been using Pi's well over 20 years. Which helped me grasp the GPX quickly, but like many still in the learning stages with certain settings and experimenting with both DD and mono.
Although my only mono is the GoldStalker 18" x 12", must say it's terrific especially the weight. Punches down deep, great coverage in tight areas but the super light weight is fantastic.
However, Being a relic hunter only use it for deeper weaker signals after using DD coils. The 11" DD is great, I find this coil to be in between the smaller Commander 5" x10" DD and the 15X12 elliptical you're considering. Have the 15x12 DD elliptical and it's great but I prefer the 11" DD instead due to weight and can run with higher gain. Although the 15x12 is deeper, the extra weight doesn't justify the added depth but that's me as others may choose the 15x12 over the 11" in a heartbeat.
Auctally, Sadly may sell off that coil in the future. Recently, Picked up the 15" Sprial DD for my heavy hitter coil on depth. And that it does and very sensitive to small finds, it didn't loose the sensitivity on the tiniest finds for being a big coil, but there's a trade off the extra windings added more weight. So, Need a hip stick to help maneuver the coil.
My favorite DD is the 10" x5" Commander DD, serious depth for such a small coil. Use it for both relic hunting and inland coin shooting old parks, especially up in gold country great coil hunting the old miner sites laced with iron. Once in a while pulls out a neat find VLF can't even get a signal, this coil has it's strong points under right conditions.
Wish I could help with more information, like I was saying I myself are still learning with the GPX.
Best wishes,
Paul