135 would probably be the most similar to 121 and so might make for an easier transition. It would probably also get you more time in IMC and in the IFR system, and good IFR skills are essential in 121 training. Disclaimer: I have never worked for a 135 operator; before going to the airlines I instructed and then did aerial survey.
Another advantage of 135 is if you get hired with your times, it would of course be a 2-pilot operation, which would give you some experience with CRM. That said, instructing is arguably an even better CRM practice. And as many people point out, teaching requires you to have a greater level of knowledge of aviation, which will be beneficial throughout your career. Nowadays instructing might pay better than being a 135 SIC.
It sounds like you don't have your CFI certificate yet, so if you can afford to you might start working on it while applying to 135 operators. You never know if you'll get a call from them anyway at their minimums, and if you do it may not be right away.
One other thing to consider is if a 135 operator makes you sign a long contract and you could get to 121 minimums sooner by instructing, it might make sense to instruct since seniority is everything at the airlines (and it sounds like 121 is your goal).
You will probably do just fine with either. And while instructing can be an excellent way to build time, it is not "the way, the truth and the light" as many claim.
Also, you might look into aerial survey if getting your CFI certificate isn't feasible and you can't get hired at a 135 operator. However that would not be nearly as gpod experience for 121 than either instructing or 135.
I hope this is helpful, and best of luck with your job search.