Well from a monthly cost perspective, the HTC Mogul (PPC-6800) is cheaper to use since Sprint offers SERO (Sprint Employee Referral Plans) which includes unlimited internet access, unlimited text, mobile2mobile, and 500 minutes with 7pm nights all for only $30/month.
Since the Blackberry requires BIS (blackberry internet service) or BES you will need to adds the Blackberry Unlimited plan ontop of your SERO plan since the unlimited data feature of SERO is limited to non-Blackberry devices.
The Blackberry 8830 WE is a nice mix of being able to travel with a
SIM card in the back which the Mogul does not offer. But as far as data speeds are concerned, the Mogul supports Sprint's
EVDO Rev A network for faster download/upload speeds than the BB 8830 WE (
EVDO Rev 0).
The name really says it all for the HTC Mogul, (pocket pc). You are getting a device similar to your desktop and laptop which means software and portability are always going to be easier on a Windows Mobile handset. More third party software, ability to modify the phone extensively and Wi-Fi make the HTC Mogul the better device IMO.
Cost-wise you can find the BB 8830 cheaper than the HTC Mogul, but you will spend more per month for service so in the end the HTC Mogul becomes a cheaper device based on cost of ownership.
The best questions that would help answer which device is best for you would be a list of your previous device history, what needs you have from the device, specific software you need to work with your phone and how you feel about device sizes.