I love my non-Ai lenses but they all needed service. The FT3 offers the ability to use Ai and Ai-S lenses which are both newer, offer better optics and are currently bargain priced.
You can use AI and AIS lenses on the FT2, or any other lens which has "rabbit ears".
So, the tradeoffs are these:
FT2:
++ uses modern batteries
++ mount and meter pre-AI, AI, AIS wide open, yay!
~~ you get to participate in the time-honored, legendary aperture ring twist ritual
~~ slightly older (1975-1977), but more plentiful (~300,000), so maybe less expensive
-- can mount E lenses, but cannot meter with them
FT3:
++ uses modern batteries
++ can mount and meter AI, AIS, E lenses wide open, yay!
~~ no aperture ring twist ritual to imbue primordial Nikonicity
~~ slightly newer (1977), but fewer made (~140,000), so maybe a bit more expensive
-- can mount pre-AI lenses, but metering is stop-down