I'm not up on the differences between 5 and 6. I just got 5 the beginning of the year because my old LR3 would not support my new 6D. That said, I had that version for over 4 years and was very happy with it. Even at the student price of $10/month, it would have cost me $480 in subscription fees over that same amount of time (just an example..... I know the subscription did not start in 2010). I'll likely keep LR5 for a similar length of time. Generally though, I'm not one to jump at every upgrade so the disc was perfect for my needs.
What I find interesting is that the link I gave you to the student price is about what I paid for the non-student version of LR5 around the first of the year. Maybe they were clearing out the 5's before the 6's came out? Not sure. But it was a great deal and I do love it.
If you are solely doing basic raw processing and cataloging, no big reasons to upgrade.
For more extensive use, "6" does have some very very nice features over "5":
- HDR and Panorama of raw files. I previously used photomatix for hdr, but that resulted in the final product being a jpeg. Now with lightroom, you get RAW hdr and panorama files. The panorama in particular, works nicely.
- Facial recognition. Nice feature for cataloging family photos, but I haven't really turned it on. Because it would take hundreds of hours to apply the recognition to my library of 50,000 photos.. but I probably should at least use it for new photos going forward.
- De-haze, just added to the cc version of "6". This really really works well in landscape photos.
- slightly faster overall performance
- More precise use of filters and brushes. you can "brush in" and "brush out" after using gradient filters, which you couldn't do before "6."
Essentially, "6" incorporates more features that you previously would have to use Photoshop and more advanced knowledge with. I suspect Lightroom 7 or 8 will start to incorporate content-aware brushes that are found in Photoshop. As they are really trying to sell Lightroom and Photoshop bundled, there is less reason for them to make features exclusive to just one of the products.