I'm of the opinion that you could give the Abbado/BPO one the slip. It's really one of his more dull Mahler performances. That said, his Chicago one would give you greater contrast to both the Rattle and Karajan M5's. If I were going to collect all those, I'd get Abbado's CSO one.
With Solti, you'll have to decide if you want the his earlier one, or his 80's remake. The earlier one was very fast and exciting, but a bit short on musical niceties. However, it's debateable if his 80's one - which is a bit slower, overall - is truly an improvement or not. Let me phrase all this another way:
With the BPO, I'd rank them Karajan; Rattle; Abbado. With the CSO, I'd rank them Barenboim; Abbado; Solti II; Solti I
Barry