I think by functionality, both are the same, although the small screen of the fields SFC will hinder tuning when using it the first time, without any help from a manual.
The VAFC has slightly more features, but if you can get a field SFC on the cheap, and have access to it's manual, why not?
If ranking tunability, I don't think either is difficult, because you're only modifying the original fuel values, which on stock setups, doesn't vary that greatly. Another thing is, on stock engines, when in doubt, just reset everything to zero, and you car will still run.. :P
However, honda ECU's closed loop systems will correct any minor discrepancies, and you might find that you need a retune after a while. It's just how it is, though it won't be detrimental to the health of the engine, just some very slight power losses, I think.
Do note that majority of people suffering from problems using VAFC or Field's SFC usually comes from wiring, either loose connections or something, I advocate the use of a intermediate harness that you can splice the SFC/VAFC into, so you don't need to butcher the original wiring, and when you need to remove it for what ever reason, all it takes it just unplugging it from the ECU, something you'd appreciate if the unit dies (rare case), or something.