Different brands will have different tensions and sometimes different brands will have a subset of different formula options that will result in more tension differences... that's four different differences in one sentence :mrgreen:
I'm currently hooked on DR Legend flatwounds. 45-105. They make for a very nice thumpy warm "old school" tone and don't take long to break in.
For flats I've tried Fender, Rotosound, TI, Pyramid, Ernie Ball, D'Addario, GHS, LaBella, and probably others that I can't recall off the top of my head. I used to swear by the old formula Fender flats, but since getting into these DR's this past summer, I think they are my new favorites. There does seem to be some confusion on the web whether Legends are wound on a round core or hex core. They may make them both ways but the two sets I've purchased (from a fresh order/shipment at a local shop) both had hex cores.
I would describe their tension as medium. Going by the apparent web consensus that TI's are low tension and LaBellas are high tension. In my experience, the gauges will only dictate tension within the limitations of a specific brand/model of string. From brand to brand, there are way too many construction variables to make the assumption that higher gauge results in higher tension.
With the DR's for example, the 105 E tension feels about the same as the D'addario 100 E and looser than their 105 E.
So I suppose what I'm saying is that your best bet is trying out different types and until you find a set you like.