Actually, we're both.

The 402s run under part 135, while the ATRs are on a 121 cert.



To answer a couple questions that don't look like they have been fully addressed:

Yes, a 402 SIC can log all the time she is acting as SIC. As stated before its not a gray area or open to interpretation, you're a member of the crew.

As far as the attractiveness of 402 SIC time to a future employer, I'd guess that all depends on who, when, and how much. Its probably less attractive than C-130 SIC time, but perhaps more attractive than C-152 PIC time, or maybe even C-130 FE time.

I can say I find flying the 402 much more challenging than flying either the ERJ or CRJ.

Upgrade time on the 402 is instant. The instant you meet the requirements for your ATP you go to upgrade training. On the ATR it could be a while... see my post about GUM FOs.