Considering that all shields mass 50 tons regardless, I tend to ignore cost and focus on function. To that end, all ships that I shield receive a set of maximum tech shields, research wise, I tend to advance the regen in step with the shields so that the regen stays 300 seconds for a full refresh.
Up until 8 hp or so, I tend to prioritise raising shield strength, but after that, I would pick more regen over more strength. Either way though, unless I am under serious pressure, the refits usually wait for both to increase one tier.