I was just in the exact same boat as you. I was completely torn between the Pocket Pro/BeBook Mini and the Sony PRS-300. For me the following just won me over:
1) SD card slot. I'm not limited to the .5 GB of onboard memory. So when my ebook library blooms to several hundred or thousands books, I don't have to worry about not being able to carry my collection with me (#1 reason why I bought a ebook reader). Plus throw in some black-and-white graphic novels and some MP3s and that 512 MB memory is full before I know it.
2) Customer support and Robert's presence here on the MR forums. I rarely, if ever, find a company that has a representative that speaks and interacts with their primary user base on a day-to-day basis. Robert's dedication here, to the product, and to customer service is above-and-beyond and I feel like I can put my trust and my wallet behind a company that listens, improves, and communicates with its customers.
3) Included cover. Honestly, if I had went with the Sony, I still would have bought a cover which would have tacked on another $25 to $40. Add shipping onto that and I'm paying $235+. For $200 + $12.50 shipping, I get a included cover and a AC-to-USB charger (something the Sony doesn't provide).
4) Look and feel. Granted, I would not have minded the smooth/metallic feel of the PRS-300, I think the rubberized feel of the PP would better suit me. I don't have to worry about my grip slipping and the reader sliding out of my hand onto the floor below. More friction = better hold.
5) Consistent and regular updates. Again, Sony has also started doing this as well, but for a smaller company like Astak (and ultimately Hanlin) to provide this level support is impressive. Being used to regular firmware updates and great product support from Cowon (I own two Cowon PMPs), I expect nothing less for an investment that's worth $200+.
My own experience with Sony has been far less than stellar when it comes to product reliability, customer support, and product support. Granted, I haven't made any major Sony purchases in the last few years, I had my own qualms about trusting yet another few hundred dollars to a company I lost faith in years ago.