JL Fathoms and SVS are both fantastic subs - I've owned both. They are both extremely impressive, especially for their size. I don't think you'd find one to outperform the other to a significant degree where 'tighter / more articulate bass' is concerned. Depending on your room, there my be significant gains to be had with things like sub location, treatments, or DSP.
Focusing only on the speaker, one sub that I have found to outperform both the Fathom and SVS subs is the Seaton Sound line. I'm running the F2 now and find it to be the best sub I've ever owned - hands down.
