I can't really comment on FedEx or UPS. The only people I know that have gotten hired over there had quite a bit of 121 PIC time. I'm not sure if they typically hire people out of King Airs and CJs. Maybe ask over on the Cargo board and see what those guys say.
If you're happy with getting hired by a major that flies passengers (CAL, DAL, UAL, NWA, JetBlue, AirTran, etc) - then maybe stick with the corporate stuff and get about another 1000 hours of Turbine PIC and I bet you'd be competitive. I know several people who have come from the corporate world and are now working at the majors.
An important question is: How long is the upgrade time projected to be at this E170 operator? SIC E170 time is turning out to be a dime-a-dozen these days. You really need the PIC time to get looked at seriously, and I don't know that "800 hours in a turboprop a few years ago" is going to hold as much weight.
I agree with the previous poster that getting more PIC time and also getting some 121 time will cover all of your bases, but I'm just wondering if that would end up wasting more time in the long run... Maybe if you just stuck with one or the other you would be better off...
At any rate, I'll give you the same advice I've heard my whole career... Never turn down an upgrade. Never turn down PIC Turbine time. That would be my 2 cents, but definitely get more input.