I guess the biggest diff between doing 50' and 25's (assuming both s.c.) is
the fact that the 50's will incorporate a turn. As for which is better when
just returning to the water I would suggest a slightly different tack. The
goal at this point should be to increase aeorobic fitness (and technique).
This is accomplished be maintining a heart rate of about 120-150 (depending on
your age, and maximum heart rate). With this in mind I wouls suggest just
about the most boring workout possible: Swim at the most even speed possible,
checking the heart rate ~ every 3-5 min, going for as long as possible with
out deviating from the prescribed hear rate.