Moving up to a bigger torch is always exciting! My last move was to a GTT Scorpion with a Regalia concentrator. If you are going to used a concentrator rather than tanked oxy, I'd strongly suggest looking into the Scorpion. It was specifically designed to consume less oxygen, for users who needed to use oxycons rather than tanks. It is much more oxygen-efficient than the Nortel torches. I started out with a Nortel RedMax, and couldn't even run the small torch, which is a Minor, at full blast on my old Devilbis 5.0 liter oxycon, and couldn't run the larger torch portion of it at all.
From the RedMax, I went to a GTT Cricket, and, using the same Devilbis oxycon, had a HUGELY improved, much hotter flame. It was absolutely amazing how much more oxygen efficient it was than the Nortel torch.
When I outgrew the Cricket, I bought a GTT Scorpion, and upgraded to the Regalia oxycon. The Devilbis is still going strong, so I have it Y-connected to the Regalia, and use them both all the time.
I prefer to work really hot, and use a large flame almost all the time. I run my Scorpion with both the inner and outer flames going at least 99% of the time. It blasts out a great flame running the two oxycons together, or with just the Regalia by itself. I make huge focal beads, and also work boro.
Too bad that you got the M-15. I also made that mistake. Was never able to get a properly working unit from good old Jack, and finally had to give up. He kept stalling me on a replacement for the original non-functioning unit, and forced me to pay shipping costs to return the defective unit, as well as shipping to get the replacement sent to me. Nice, huh? I had to pay shipping 3 times! The second one never worked well enough to use except in dire emergencies, but I literally could not afford to send it back to him. It was obvious that he wasn't going to send me a good unit, and told me outright that he didn't have the $$ to refund my purchase, so there was no point in paying shipping again to return it.
My all-time favorite oxycon seller is Kimberly at Pyronamix.
http://www.pyronamix.com She is incredibly knowledgeable about the various oxycon brands and models, sells at competitive prices, and offers good warranties.
Good luck with your upgrade!
Birdy