You're going to love the M1. Great regs. As it turns out, I own ST1's, M1's, and one Apeks reg. Now that I think about it, the 1st stages on my rebreather are Apeks, too. All of them are great. I *DO* feel like the atomics breathe better than the apeks...but in truth, I'm sure it's just due to how all my regs are currently tuned, and that could easily change next service I get. ALL of them breathe awesome, though...I'm splitting hairs no matter how I look at it.
My only recommendation, if you have not already made your purchase, is that the DIN may be a better idea than the yoke. Having a yoke adapter attached is not really a disadvantage, and it's nice to be able to attach your reg to any type of tank.
I started using only rental tanks, quickly transitioned to having my own, and now that I am doing predominantly tec diving, I only use DIN connections. My DIN regs can all be put on yoke tanks with adapters, but my old yoke ST1 doesn't tec dive with me, as I can ONLY use it as yoke. At least you can always use it on a DIN tank that accepts the spin in adapters that make them yoke tanks...and rental DIN only tanks aren't common in my neck of the woods. Rental tanks with spin outs are becoming more common, though, as a lot of newer valves come that way -- so I can use my DIN regs on a lot of rental tanks without the yoke adapters being needed. Even when doing single tank dives, I prefer the DIN connection vs the yoke because of the slightly lower profile. In regards to tec dives? Who knows what you will do one day...I never suspected when I started diving OW that I would one day be doing caves and using a rebreather.
If you DID already make your purchase in a yoke config, I wouldn't worry about it. The yoke M1's will treat you great! And if you ever want to convert them to DIN, it's MUCH cheaper to do with the M1 than it is with the ST1. DIN conversions can be had for around $50...the ST1 stainless DIN conversion is $200!
