Based on a review in the May 1980 issue of Modern Recording, I would chose the TDK. Maxell always advertised like they were the best and everyone knew it; but according to this review, their UD-XLII was only average as high-end cassettes went, while TDK SA came out on top. In the high-bias tapes, seven were compared. Here's how TDK SA and Maxell UD-XLII compared.
R/P response @ 0VU record level:
TDK 24Hz-9.8kHz, Maxell 24Hz-9.4kHz
R/P response @ -20VU record level:
TDK 23Hz-25.6kHz, Maxell 23-25.0kHz
Maximum REC level for 3% THD @ 400Hz:
TDK +6.0, Maxell +4.5
MOL @ 10kHz:
TDK -2.5, Maxell -4.0
PB sensitivity @ 1kHz, 0dB REC level:
both -0.5
PB sensitivity @ 10kHz, -20dB REC level:
TDK -18.5, Maxell -20.75
SNR, "A" weighted, Dolby off, Re: 3% THD, 400Hz:
both 56.5
SNR, "A" weighted, Dolby on, Re: 3% THD, 400Hz
TDK 64.5, Maxell 63.5
THD @ 0VU REC level, 1kHz (in %)
TDK 0.9, Maxell 1.05
The machine used for the tests was a Nakamichi 1000. In all the areas except the two where they were tied, the TDK was better. In fact, TDK was the winner in the contest of all seven tapes.
TDK SA is still being made, but TDK's top-of-the line today is SM. I recently bought a bunch of SM, but I can't say I've used it enough yet to comment much from personal experience with it.