Both the 3.2 amp (V1.5) and 4.0 amp (V3.0) units perform beautifully. The implementation of resistance checking, voltage setting, and battery remaining graph is particularly smart and ergonomic (a single button click gives all three readings).
The difference is in cap design. V1.5 uses the original-style "nubby" end caps made of plastic-covered metal. V3.0 has the sleeker all-metal caps with the top connector being a flush 510 style. If you want the higher amp limit combined with the 510 eGo-style top connector with the spring-loaded center pin, you'll have to wait for awhile. Currently, those "Lambo" bodies are available only with the original V1 electronics (2.5-amp limit, no resistance checking).
I didn't (and don't) care about the bottom-cap, top-connector design. What I wanted was the higher amp limit and resistance checking, so I'm happy with both my V1.5 and V3.0. I even like the fact that the cap designs are slightly different (I can easily tell the units apart), although I did sand off the "nubs" on the V1.5 bottom cap. Makes it more pleasant to hold, in my opinion.