I've got a couple different Milwaukee's, Here's my findings on using them.
M12 3/8 non fuel,
Not much torque, great as a nut runner. Big time saver when dealing with hardware already loosened manually and for assembly work. A 1/4 drive kit can be installed in a spare ratchet and saves using an adapter at times. It is bulky in that application but it has been useful for me. The 3/8 I'm using currently is from '17 and showing it's age some, it is a daily use tool though.
M12 3/8 fuel (1st gen?),
More torque then the non-fuel version, about 35-40 ft lb vs maybe 20ft lbs, it could potentially break off a 6mm bolt (or feels like it anyway). Definatly bulkier in the head, I've not run into any clearance issues yet, But I'm mostly using it for brake jobs at the moment, some other stuff too, but no under hood work to speak of. I've only had it about 6weeks & got it used from the Matco guy.
The 3/8 fuel is similar in torque to the Snap-on unit the boss has. S-O is less bulky but the trigger is smaller.
M12 1/2 fuel,
Great for larger bolts (12/14mm) that are taking a bit of effort to remove (locite, rust, ete..), I think it could potentially snap off an 8mm bolt. Bulk & size havent really been a problem. Main use is caliper bracket bolts and some suspension. Kind of front heavy and a bit of a club. Got it as a gift from work last Christmas.
Summary,
If I was buying these over again, I'd go with the non fuel in 3/8 and also get the 1/2 drive fuel. I was dubious as to how useful the 1/2 drive unit would be, but it actually is very handy at times. A 3/8 fuel could be the best of both worlds depending on how you plan to use it.
The Snap-on & Matco battery ratchets are nice, but more expensive then I think their worth for my purposes.
I also have a non-fuel 3/8" and I like it alot. Crack the bolt loose using it as a ratchet, then zip it off. Got mine for ~$80 bare tool on amazon.
To the OP:
Milwuakee non-fuel is lower torque, but more RPM. Fuel has more torque, but lower RPMs. If you are using it on something with a lot of constant friction (nyloc nuts, rusty bolts, etc), fuel might be better. If you are using it on applications that have low running torque once broken free, the non-fuel is cheaper and probably better from my perspective.
I would agree.