Type M vs Type L copper

My understanding is that plumbers use Type M for residential and Type L for commercial.

Why the difference? The cost difference seems miniscule. How much life can one expect on Type L over Type M? I wouldn't have minded paying a little extra for Type L on my bathroom but didn't know about this difference at the time. But I don't think plumbers offer their customers the option....

Any reason DIYer should stay away from Type L?

Local codes apply. Some areas require only L; others allow M in 1 or 2 story residential. L costs almost twice as much, but in dollars it is small portion of the cost of house. L is of course much better if there are any water chemistry issues or water velocity issues.

l or m

One difference is that the thicker wall makes it harder to cut and it also creates a "ridge" at the cut because of displacing more metal while cutting. That can make assembling the system more frustrating since it makes the fittings harder to slide onto the pipe, sometimes to the point of having to remove the ridge with an outside reamer.