Assuming eisenach means the live version with the LAPO (mine is coupled with Appalachian Spring, and Bernstein's own Candide overture), I have both the above and have just listened to them back-to-back, something I've never actually done before. Both have their merits. The Bernstein is, as you'd expect, much more heart-on-sleeve and impassioned, every phrase sobs at you. It's also almost exactly a minute slower than Slatkin (10'02 vs 9'07). If you see the piece as a mourning piece (rather than an impassioned love-song), the Slatkin is more inward-looking, stiff-upper-lip solitary grieving. Bernstein also has the much warmer, fuller sounding orchestra, Slatkin's seeming a bit thin to me, although the interplay between the different lines does then come through clearer. I also actually think the sound for Bernstein is better than for Slatkin.
Up to the central climax, I'd say Bernstein wins hands-down (and there's a rather obvious edit in the Slatkin leading in to it) - Slatkin's approach seeming extremely mechanical to me. Following that climax, though, Slatkin gives an absolutely enormous general pause, and then continues with the final phrases imbued with quite heart-rending desolation. I still find the Bernstein much more involving overall though, as Slatkin has a lot of ground to make up with that wonderful ending - so that would definitely be my recco.
If you want SACD, then it looks as if it has to be Slatkin/St Louis again, but in a stragely mixed program of string music on Telarc not EMI, so it's almost certainly a different performance. Probably will have beeter sound that either the EMI or DG. Looks like you can actually download clips from
http://www.telarc.com/gscripts/title.asp?gsku=0641&mscssid=LSLGB6X23X319PXQFPQHMPMJ41VSC9R6 if you want to check the performances out. Unfortunately that clip's too short to get any feel for the majority of the performance...it does sound very similar in style to the EMI one though. I also have a choral version on SACD (Flummerfelt and the Westminster - USA, not London - Choir). It's nice enough, but doesn't do anything for me at all - probably best avoided.