bananagrams wrote an answer about Arc'teryx Acto MX Hooded Fleece Jacket - Men's on February 4, 2013
i havent used the nanopuff myself but ive heard good things. i personally get super warm super fast, so im often just wearing a merino baselayer + the R1 on approaches, add the acto mx or atom lt while climbing, and carry a nice warm belay parka (just settled on the Rab Neutrino) for stops.
i find the merino layer + R1 + Acto/Atom LT will keep me warm in just about any weather if im moving (unless its super windy, in which case i put on the alpha SV).
if i were you, i'd probably keep the venta mx, and add a heftier belay jacket (the atom SV, Rab neutrino, or patagonia DAS parka, depending on your down/synth preferences) for stops. i wouldn't use the atom lt or nanopuff as a belay jacket unless it was pretty warm out.