Finding a perfect piano sample is still the holy grail of sampler science. I have a Roland RD-700 SX which has really decent Sounds, with one bright and one mellow Grand Sample Bank which is tweaked to many different nice piano sounds. But the sample loops are too short. Hold a key for a longer time - and you'll hear the sample looping, fading much too quickly, like you said.
Compared to this, Yamaha has very nice samples (which i must admit, even though i am definitely no member of the Yamaha tribe). Since the Logic Samples are from Yamaha (or so it seems), they are really good as well. The Bösendorfer is only that nice because it's a channelstrip setting. It's not the samples alone, AFAIK.
However: If you want to have a real piano sound, record a real piano 🙂 If you want to spend 300 Bucks, buy two nice condenser mics for stereo recording and search a location with a good concert grand.
If you just need a standard piano sound in a pop arrangement, Logic Express' piano samples are all that you need. Tweak it with Platinum Reverb for ambience, let it run through a soft compressor, and there you are. If you want bigger quality, pay bigger bucks. You may have a look at the Piano from Native Instruments as well. There is a Bösendorfer sample included, and you pay "only" 200 Bucks for pristine sound quality.
Fox