Lots of conflicting advice here, ultimately you will have to make the decision. A few pointers:
- MCs are inherently inbred, there is absolutely no problem switching from one to the next, as long as you can explain well and there are a few years in each (ie you haven't moved every few months or every year)
- Most of the consultants I met moved due to £££ and promotion. When I first met them they would talk about how much better the place we were at was and what an amazing environment it was etc etc. Once I got to know them, they would explain its all much of the same but to get a big pay rise / promotion the easiest way was to move consultanties and hence voila, here they were. Yours seems no different
- Don't worry about Big 4 vs other consultancies. If you were at MBB, then Coolio or BEP could provide better advice but going ACN to one of the Techs is no issue. If you make sure you are top quartile performance and keep a good network, you will be able to move between MCs easily enough in the future
- Ultimately, decision is yours. Make sure you get a nice fat payrise and a solid promotion, alongside nice projects and then you won't have anything to complain about.