Depends on a couple things. First, do you have premium items - and if so, which ones? I’d go for the type with the best prem items (in most cases). Additionally, I find that building mechs that have the same optimum range helps a lot in 2v2. That is, all mechs short range (like most physicals), or long range (like F2P heaters), etc. If you can get both (or all 3) mechs you use like that, then you’ll spend less time moving around and more time killing in PvP.
Also, a general note: phys tends to be easier to make and use; heat is harder but it pays off if you can make it work. Just my opinion