Don't just concentrate on the engines , the 1960s +4 and 4/4 are very different cars.
The TR engined +4 is more powerful , and the engine develops greater torque at lower revs , so it pulls well in most gears. The +4 is equipped with the Moss Gearbox , which has 4 speeds and no synchro on 1st. It is a delight to use when properly set up, but can be a real handful when worn (and most are these days), it will never have the rapid change of the Ford boxes in the 4/4. The rear axle ratio is normally 3.73:1 so cruising revs are lower than in the 4/4. It is also considerably heavier than the 4/4 so much less "chuckable" in the handling department. Today most parts are available for the engine (at some very high prices) the box (at eye-wateringly expensive prices) and even the axle.
The Ford engined 4/4 would be a 1340 or 1500cc non-cross-flow unit in the 60s . It will rev easily (more so than the +4) and enjoy a higher limit. The gearbox will be a delight to use (especially with the 2000E box) and encourage rapid changes. The rear axle would normally be 4.1:1 , (or possibly lower) so cruising will be at higher revs and can be wearing on long journeys. Being Ford derived the drive train parts are normally much less expensive and more freely available.
So in summary if you want a car with easy cruising and less nimble handling go for the +4 but be prepared that repair bills could be higher.
The 4/4 is much more nimble , lighter, enjoys easier handling, and likely to be less expensive to maintain but less good as a tourer.
As ever with Morgans always buy on condition alone , much better a really well sorted car with lots of miles and a good record of services and parts replaced than a garage queen that will take a walletfull of cash to sort back to roadworthy condition even though it might look the part.
1999 +8 , Indigo Blue.
Ex-John McKecknie/Mike Duncan 1955 +4 racer.