Late, but it really depends. Some schools (ie: UCLA off the top of my head) specifically want you to have an MLIS and will ask you to do additional coursework equivalent to MLIS competencies if you don't have it.
Alright, I figured I'd kick us off - who's applying this cycle and where?
I'm undecided about whether I'll work the field or do a PhD afterwards, but I have an MA in Art History, so I'm applying to places that specifically have a strong emphasis on architecture or visual culture materials, have a thesis or MRP, and are in a city with strong cultural ties.
My list, in order of interest, is:
1. Joint MI + MMST at U of T
2. Film + Photo preservation and collection management at Ryerson
3. Dual MAS/MLIS at UBC
4. McGill MiST - Project
As far as your language proficiency goes, many schools require 2 languages outside of English... so how well do you know a third language? You'll want to take a look at the requirements for the schools you're applying to!
I don't think there's a "perfect" answer - it's whatever is right for you.
I can only speak to my own experience: I'm from Canada where it's not "the norm" to do your PhD right after your BA. We all do M(something) degrees before moving on, unless we're studying in the states. That being said, the PhD is less lengthy here than in the states (3-4 years).
I do know that doing my MA really helped me hone in on my research skills - graduate seminars and expectations really are different than in your bachelor's degree. I'm applying to PhDs now and I feel like the work that I did in my MA absolutely helps make me a better scholar and more competitive applicant. If nothing else, I now have an extra two years of field knowledge, which is invaluable to me. I can confidently say that I have the background I need to pursue the research project I want to pursue and it's because I did that foundational work in my MA.
That being said, if your senior project is related to what you want to pursue for your doctoral research, I definitely think you'll get some of that same experience. Just be sure to really do your work when it comes to understanding your subject area and that you do a more comprehensive lit review than would probably be required. It'll help you out a lot in the long run!