It really depends what you want to study apart from GOVT.
- In the BA, there are few subject restrictions and you can do whatever you want within the arts faculty.
- In the BPESS, alongside the core GOVT major you do a compulsory sequence in political economy and sociology/anthropology, and then for your second major you can do any subject in the arts faculty (including ECOP or SCLG or ANTH if you want, or you could broaden your perspective a bit more by doing history or cultural studies or something, or a language).
- For the BIGS, alongside the core GOVT major you do the compulsory sequence of specific International and Global Studies units (Power and Money in Global Society, Global Culture and Society, Transnational Spaces and Networks, Current Global Issues) and then for your second major you can do any subject in the arts faculty, which again could be one of the other BIGS core subjects or whatever else you choose.
If you're planning to major in GOVT no matter what degree you do, really the only difference is in the compulsory sequences (or lack of one in the BA). Though I'd also note that first year is less flexible in the BPESS (you do GOVT, ECOP and SCLG/ANTH and only get to pick one other subject area) whereas the others give you more ability to try stuff from across the faculty and see if you discover an interest in another discipline.