Presumably if you're only 7 months in and you've already gone through an entire interview process for another job that's a hint in itself?
My view would be yes, for all the reasons below (including a promotion by the looks of it), and to get the name on the CV. Usually it's the other way around - get big brand then move to boutique. But as someone new to consulting it strikes me as logical to go to the 'best' firms. Another consideration is it depends on your specialism - is your boutique considered a leader in its field? If so, that's a reason to stay. If not, then jump to a bigger firm.
Worth considering what is important to you - the culture of the big firms is vastly different from industry & indeed boutique and it doesn't suit everyone (I speak as ex industry working in big firm).
