I would say PIC in a regional jet under 135 is at least as valuable as SIC in a regional jet under 121.
A lot depends on the type of flying you're doing. Are you flying 10 hours per month or 100? What type of support are you getting from the company? (i.e. do you have a 'dispatch-style-flight-follower' or are you making ALL the decisions about flight planning/fuel/alternates/filing flight plans--you can use this as 'experience credit' in an interview)
If you are accumulating TPIC in an ERJ at a decent rate, you can sell that very well as equivalent to, or maybe better than 121 experience.
There are many variables in the equation, but one possible track to a major could be 1000 hours of experience in your 135 PIC position, and depending on the rest of your application, straight to a major. If you have weaknesses in your application, a couple years as a 121 SIC with a LLC or with a regional should make you marketable to a major since you proved your PIC mettle with your 135 job. 1000 TPIC isn't the silver bullet it used to be, but I think it might minimize the need for 121 experience, particularly in todays hiring environment.