No idea which would be the better; best to weigh up the differences between the two, or talk to a Rega dealer who might have heard both. Additionally, most dealers will undertake a service too (circa £35) so don't be surprised if they offer this at the same time as doing the motor upgrade for you.
I almost went for the Rega upgrade (which from memory makes a good difference over the cooking motor) but as the P3 is now on top of the wardrobe and likely to be sold off soon, I've spent my money on ageing Japanese amps of late instead....
Come to think of it, I might even have to rename myself - recordless_spot perhaps, or "_spotless" for the cleanliness obsessed!