Hey RG (Really? I didn't think there would be any difference in sound really from digital to digital.), this is why I crap on about it. You have to "dither" between different digital formats, typically throw bits into the bit bucket (in a slightly intelligent way). There are various dithering algorithms (e.g. MP3 where to rip a CD and then throw 80% of it away turning it into a MP3 file (MP3's have other issues to)). MP3 is a serial (streaming) format, WAV is a word based format (each sample is stored as a word (16 bit, 24bit, 32bit etc). The bigger the word (bit depth) the more dynamic range and finer granularity of each individual sample. While sample rates (44.1, 48, 96, 192 khz etc) make a difference (more samples more accurate), after 48 khz the bit depth make more difference. Hence 48k 24bit is a reasonable sample rate to use. The least significant bit of a 24 bit sample is -144db (not really audible), the LSB of 16 bit you can hear.
So if you record at a decent sample/bit rate, then need to turn it into another format (e.g. CD) you have to dither it down and throw out detail. Given the much higher granularity of high quality analogue (a decent mastering machine will go out to 80khz (not chop off at 20khz)), it's a way of doing the dithering more naturally and gives better results, but is much more hassle so depends how much you care.
The closest we get to a decent mastering machine in digital is sony super audio, it's serial like MP3 but samples at 2Ghz and goes flat to 100khz. MP3 sample at various rates, best the Lame codec does IIRC is ~356k and typical internet MP3s are 178k (or there abouts).
Destructive record is a protools option so when you record over a track it blows away the previous track a reuses the space (same as if you recoded on multitrack tape). If you don't select it, PT keeps each take as a separate file (takes more space) which you can recall if you want. When I started using PT, not only was I more familiar with tape, I also suffered a lack of fast disk space, I'd fill a 10,000RPM 10GB drive (very large for back then BTW) and then have to burn the files to CD to record new songs.