They are both nice stores to be sure. Your time constraints combined with
wherever you’re staying while in NYC may have more to do with your choice than
anything else. Garrett Wade is closest to lower Manhattan and can be combined
with a walk over the Brooklyn Bridge if the weather is nice. TFWW requires a
longer subway ride to Greenwood Cemetery, which is also worth a visit in its own
right, weather permitting. The “N” and "R” trains however, have always been a
slow ride in Brooklyn, recent news articles about the subway’s woes
notwithstanding. I could never make a trip to TFWW from my office in lower
Manhattan at lunchtime that could approach anything remotely like 90 minutes.
YMMV

John