The featherbed is a long frame, a pre unit fills it better. The best looking is when the long chain case is used, that really looks like it was meant to be!
There are technical bennefits too, the pre unit enables you to mount the engine a long way forward, which is good for weight distribution and handling.
It also allows the gearbox to be further back, which is good for correct alignment of the rear wheel to swinging arm to gearbox sprocket centre lines.
However, the unit engine can be mounted to perfectly negate the above issues, it also have the benefit iPod having much stronger crank cases and even stronger gearbox shell.
Dave Degans (Dresda) prefers until engines because of the above. He sells engine plates etc to make it easy to get it right.
Finally, although many components can be retro fitted to different Triumphs IF you know what you're doing, why bother? Just buy a later T140 motor and you have a 5 speed box, 750 top end, etc.
So... The easier / sensible choice for a fast motor is unit. The more aesthetically 'correct' version is pre unit... Both will work well. Its all down to personal choice really.
PS, I know someone selling a brilliant 750 pre unit Triton, squished head, belt drive, Maxton forks and shocks, lots of Dresda work, etc. PM me if interested, bike is in UK.