Hi HB and
to the forum.
The choice between front or rear motor is usually decided by ease of fitting and the rider's personal preference.
If your rear wheel is currently fitted with 8 or 9 speed cassette gears, it is not possible to simply transfer this over to the Magic Pie.
Therefore fitting a front Magic Pie would be cheaper and easier to install, whilst also allowing you to retain all of your current gears.
This is not usually a problem, provided you have steel forks, as their drop-outs are usually strong enough to withstand the torsional load being passed through the wheel's axle, although fitting a separate torque arm would be advisable.
Although a front mounted Magic Pie should give a better weight distribution, I much prefer a rear mounted motor, as it provides more traction on the driving wheel for hill climbing on slippery surfaces etc. and a much shorter cable route for the current flowing between the battery and the controller/hub motor.
Regen
(which is operated by the power breaking switches on the brake levers) works just as well on the rear as it does on the front, and there's less chance of losing the front end
(and your steering) while braking into corners at speed on loose surfaces.
A rear hub motor is also much more fun for pulling wheelies
, and the extra weight on the rear should help prevent you from flipping over the handlebars when you brake hard with the front brake.
In my opinion, a rear mounted motor is safer.
The choice of throttle is sometimes decided by necessity, rather than personal preference.
If you have a twist gear-change, then a thumb throttle is probably a lot easier to fit.
If you have the lever type of gear-change, then the twist throttle is probably better, but I'm used to riding motorcycles, so the twist throttle seems to be much more natural for me.
Regarding the choice of rack, if you have rear suspension on the bike, the seat-post mounting should provide a more comfortable environment for your battery pack, although I suspect that a fixed rack would safely carry a lot more weight than a post-mounted one.
This should help you to make your own decisions.
Alan
P.S. I would never describe myself as an "expert", because an "ex" is a has-been and a "spurt" is a drip under pressure!