PCM gives you the colour screen, which is used for trip computer, integrated phone operation (if fitted), CD-autochanger operation (if fitted), radio operation, sat nav operation.
IMO it is better to have it than not. It will make resale easier. I read somewhere else it is worth a grand on the price - not sure if this is true. I prefer the look.
Some aftermarket sat navs may be better, but it's original equipment and fully integrated with the other gubbins.
As for depreciation - you lose 5K if you drive it out the showroom, since this is the difference between trade and OPC retail. The biggest hit (the 1st year) is already taken care of, but I would still guess you'll lose 5-10K over the next 2 years. So if you pay 50 now, it'll be worth 35-40 in 2 years. I think hoping for more than 40 is optimistic.
It's hard to be sure, but I don't think there will be a big shortage of them to push the prices up, and the economy is not so certain. I think you need to be realistic/slightly pessimistic, then if you can do better you'll be smiling.