AG Gandiva Jishnu's will actually be less impressive than a Fail-Not owner if Arhcery is your thing....the difference though is the AM3 that pushes WS dmg out with every RA. That being said, not only is AG Gandiva prohibitively expensive, so is gearing for STP Barrage, +CHD Jishnus, Snapshot/Rapid Shot preshot, +Crit TP set.
Ultimately, what do you enjoy? They are all viable options. I think Verda's spreadsheet puts them in the following order;
1. Gastrephetes
2. Armeggedon (but you will have more difficulty True Shotting, this significantly lowers spreadsheet DPS)
3. Gandiva (realistically this is prob #2 due to distance)
4. Fomalhaut
5. Fail-Not
P.S. I am ignoring the fact that Gandiva pigeon holes you into physical damage. Can be a deal breaker, so keep that in mind.