Both models used to race in local club races. Both seem fast. One metric that should be stressed is actual livability, i.e. below decks.
When we spent a year, 1994, seeking our next (last?) boat, we were focused on boats in the 30 to 32 foot range. With several friends being very happy with their J-30's, we really wanted to like that model!
Problem was... even with the later cruising upgrade to the interior it would never have enough head room for a 6'2" guy, and even if I could adjust to the main cabin, the head compartment was way too cramped. When spending two to four weeks on board, this stuff is kinda important.
As a secondary concern, we both wanted a cockpit with backrests for the seats. While I would agree in general with the advertising from J Boats that it was designed to be a "fast cruiser" (or words to that effect), we finally had to back off for the above reasons. Great sailing boat and I would still recommend one to anyone, with those caveats.
Having spent a prior decade cruising and racing a Niagara 26, we really wanted to continue with the transom rudder, aft traveler, and fractional rig. We just needed more "living" room inside.
Nothing wrong with the Sant 30 (and similar designs) but we knew we wanted another boat with a large main and manageable fore sail sizing. We liked the smaller chute you get with a fractional rig a lot.
As fate would have it we searched for a year and finally found our Olson 34. Rig is balanced between J and E, but is of course a mast head rig. Everything's a compromise, I guess.
So, please let's dial down the conflict level. No one here is going to have the final definitive word on any comparison, anyhow. Take it all with a grain of salt, especially what I say!
Happy new year, and fair winds to you all.