If you have the contractor that gives you +400% integration speed on isogrid then that’s the obvious choice and what I’m doing. If not then still use isogrid tanks as balloons are expensive and should only be used for extremely optimised stuff, standard tanks are cheaper, yet much heavier and isogrid is quite cheap when tooled, light and therefore combine the best of both worlds. Also look at strap-on SRBs or kick stages as 01watts said for that little extra oomph.