Well abacab and kk1, it's complicated! PGMusic have a sale on the launch of their yearly updates every year around this time. Which package to buy is always a commonly asked question and a tricky one because it depends on your use case.
For a start they no longer do the EverythingPak (
UPDATE: PG support have now told me that the current equivalent is UltraPak+) which I bought in 2014. That gave me every RealTrack (RT) available at the time and every year I have upgraded via the PlusPak, 49 pak (this and bonus only available when you purchase during the sale) and XtraPaks available as they are released and at all times (the new one is Xtra Pak 6). So I should have every RT except for the ones available pre 2015 as bonus/49 Paks. These cover all kinds of styles, some of which I have never used and I'm not likely to, but it also gives you a wealth of options (my BIAB 2018 currently lists 6,068 styles in stylepicker).
You can be creative with it. For example, once I have a basic song structure in place I will try lots of different/similar styles and sometimes use particular RTs from each version, or swap out RTs used in a particular style with others. You can bring all of them into CbB and have a lot of fun mixing and matching (Realband offers similar things but I just use CbB as I know it). In CbB I will often use some BIAB, some VSTis and blend them with real instruments which I have played and vocals.
I find myself using the RTs more than the MIDI although I have had some success with the MIDI and even exporting RT based songs as MIDI (there are dependencies though). For a lot of folk it's all about the RTs.
Have a look at that table at the bottom of the packages page abacab as it shows you what is included in each version. A good place to start if you don't want every RT but want a step up from Pro would maybe be the MegaPak. If you want all the currently available RTs then you are looking at UltraPak upwards. I have never found the need for the AudioPhile version but maybe one day :) Be aware of the disk space needed for all of those RTs - my current BB folder is around 145Gb which is largely due to the RTs (it was around 110Gb when I first invested in BIAB).
I'm waffling on but hopefully this helps in some way. Also worth mentioning that there are a lot of very helpful, knowledgeable and friendly folk over on the PGMusic forums. Some of them have been using BIAB since the DOS versions so know it inside out and they helped me a lot when I got stuck.