First, I'm assuming a continuous route, so all my decisions will be based on capacity per base trip time (cap/btt). Trade vs. transport makes no difference in the final outcome. Secondly, I'm comparing things assuming normal progression. I'm not running numbers on level one ships with level 3 upgrades, for example.
Level one ships: always use 10 capacity.
Level two ships: always use 10/20 capacity
level three ships: at first, use 3x20 capacity. Later, use two 30 capacity and one 85% sail
L1 capacity upgrade is 50 cap/btt
Using a L1 sail means getting (40/.95=)42.1 cap/btt
Cargo hold is the clear winner.
Level two ships have more options:
Two L1 capacity upgrades means 90 cap/btt.
a L1 cap and L1 sail means (80/.95=) 84 cap/btt
two L1 sails means (70/.95/.95=)77.6 cap/btt.
Capacity is the clear winner.
Level two ships, level two upgrades:
two L2 capacity upgrades means 110 cap/btt
one L2 capacity and one L2 sail means (90/.9=)100 cap/btt
two L2 sail upgrades means (70/.9/.9)=86.4
Again, capacity wins the day.
Level 3 ships, Level two upgrades:
three L2 capacity upgrades makes 190 cap/btt
two L2 capacity and 1 L2 sail is (170/.9)=188.9 cap/btt
one L2 capacity and 2 L2 sails is (150/.9/.9) = 185.2 cap/btt
three L2 sails is 130/.9/.9/.9 = 178.3
Full capacity again
Level 3 ships, level 3 upgrades
three L3 capacity is 220 cap/btt
two L3 capacity and 1 L3 sail is (190/.85) = 223.5 cap/btt
one L3 capacity and 2 L3 sails is (160/.85/.85) = 221.5 cap/btt
three L3 sails is (130/.85/.85/.85) = 211 cap/btt
Again, these considerations are only for continuous routes! If you play frequently and use a lot of small, one-time trades, then having sails may be more enticing for you, to free up ships more quickly.