If you plan to install an autopilot, then electric trim would be a plus because the autopilot will automatically trim the plane as needed for lateral/longitudinal imbalance, and if you plan on flying IFR, I recommend a 2 axis autopilot. An autopilot and lateral electric trim can be added later, but for pitch trim, the mod may be a bigger challenge.
If you don’t need an autopilot - I don’t, and I fly VFR cross countries whenever I want - then manual trim is extremely easy to use, almost failsafe, and you can visually see your trim setting all the time. Roll trim installation takes about 2 hours to install. Pitch trim takes a little longer to install, but it is a simpler install than electric trim, I’ve installed both several times.
If you are building your airplane to sell - in other words, building it for someone else, then installing an electric trim system that is compatible with a connected autopilot, whether you install it now or not, might be the game changer for someone interested in buying your airplane. If you are building your airplane for you and you don’t need an autopilot, then lighter, simpler, less expensive, may be the way to go. It is for me on this, my 5th RV. My others have all had electric trim, electric flaps, and autopilot. This RV4 I have now has none of that. None of my manual systems can fail, unless I do.
On the cross countries I’ve flown, my manual trim system works very well. If I was flying IFR in this bird, I would want an autopilot with a connected electric trim system. I’ve had that before, but found it cost me several thousands of dollars per hour to have that capability, because I didn’t fly IFR in my RV8 that often, therefore, not worth it to me.