This topic comes up frequently and you should be able to find plenty of opinion in posts within the last few years, and those opinions will vary.
I've used the audiophile version of BIAB for many years and highly recommend it with the following understanding:you will need good equipment, a good listening environment, and good ears to hear the differences. In other words, if you listen to MP3s in a car and you have long lost your high-frequency hearing like some musicians, you will not be able to notice any difference.
The 'regular' version uses .WMA files. The specs on these are often 128 Kbps, some less so. The sound seems to me to be comparable to a 198 Kbps MP3. The compression is about 11 to 1, so they are the same smaller size of a 128 Kbps MP3 but sound better than that.
Some instruments (such as acoustic guitar, acoustic grand piano, and cymbals) show compression artifacts more than others and thus are particularly better sounding in the audiophile version.
The audiophile version uses .WAV files of 44.1, 16-bit, or in other words CD quality. Because they are not compressed, the .WAV files will take up about 11 times more space than the corresponding .WMA files (and you still have the complete set of .WMA files).
The audiophile version runs slightly faster, which might not sound intuitive, but it does so because BIAB doesn't have to first uncompress the .WMA files. The way BIAB works is, if there are .WAV files available, it uses those; if there are none, it uses .WMA files, uncompresses them, then runs. This scheme makes it possible to have a combination of .WAV and .WMA files in your RealTracks and it works fine.
Be aware, the annual upgrade price for the audiophile version will be more.
At this time, PG Music ships the audiophile version on a very nice external drive that uses USB 3.0.
The difference in quality between the audiophile and regular versions seems to be getting less over the years. In the early days of RealTracks (2007), there were easily recognizable differences. I suspect improvements in the BIAB program using the Elastique algorithm have made this difference harder to detect, but that's just a guess. This year for BIAB 2017, PG Music has re-recorded a number of RealTracks for better sound.
Regardless of whether you have the audiophile version or not, the farther you stray from a recommended tempo, and the harder the Elastique algorithm must work, the more you risk hearing artifacts. Also, try unchecking some of the speed-up options in Preferences, RealTracks. I have a fast i7 and I even turn some options off to get the best quality I can for my final mix.
If you are producing commercial recordings, the audiophile version also makes a difference because noise in a mix is cumulative. If you have a little noise on each track, but add many tracks, you may notice it more. Therefore, my guideline is, if you are doing commercial work, you should get the audiophile version. For anyone else, it depends on how much you value having the best sound you can get. For me, as a composer, I get inspiration just from great sound, and that is reason enough to go audiophile.
If you have just purchased BIAB and like what you hear but want the best possible sound, call PG Music Sales and see if they can work something out to upgrade to the audiophile version.
All comments about BIAB also apply to RealBand, which uses the same RealTracks.
PG Music has not approved (nor so far, disapproved) of anything I have said here. As mentioned, I welcome and will make corrections.
Devoted BIAB User since 1994