well the serial version adds fuel in 500rpm incriments and the usb does it in 250rpm incriments
they both are rather easy to use the most difficult part as for the serial port is to find a laptop that still has the 9 pin (i think) port in the back. i have an older computer so thats really no issue plus i have an older laptop that has it so no issue there either.
and if you get the the 9v power supply you can load the maps from the comfort of you house but you have to uninstal then re-install the pc3
to check it or you can just do it from the bike with the ignition turned on but you may run your battery down playing around with it.
the serial port version known as the pc3r
has the ignition module built in so you can adjust timing as well as fueling were as the usb version doesnt and you have to pay extra for a seperate module and i think the cost is over 200 maybe closer to $300 not really sure
the usb's best quality in my mind is that you can add the ldc monitor,quick shift and their ignition module plus you just can connect disconnect with the usb which takes seconds vs the 10-15 seconds it takes to screw in the serial port.
now if you dont plan on adding the lcd or the quick shifter and just want a way to get more performance without jumping all $300 plus on a usb version the the serial port is the way to go its about half the price and you get near the same performance
so in the end it is up to you.
as far as running a 954 pc3
on a 929 yes you can and all you really need to do is be familiar with cutting and pasting.
you take the valuse from a 929 map of your choice and copy all the vaalues and then you paste them in a zero or empty 954 map and save it as something like (929-954 scorpion slip-on)
now the 954 has a lil higher rev range than the 929 so you will have some empty values in the 11,500 to 12,500 range which is fine because the rev limiter wont let you go that high any ways.
one good point is if you do decide to go with the 954 power commander and for some reason you 929 motor takes a dump on you and you decide to up grade to a 954 engine you will all ready be set vs having to sell your current power commander to get a new one to make up for the rev range that the 929's doesnt give you.
and i am running a 954 power commander serial port version on my 929 with a 929 map with no issues, and if i can figure it out anyone with an incling of computer know how can figure this out.