Screens.
That's like allowing a SFB player with a Fed CA to be able to allocate (if I remember my shield strengths correctly) 30+24+24+20+20+20=138 shield boxes to any side he wants. What player is NOT going to allocate most, if not all, of the shield boxes to the two or three sides he is almost certain that will be facing the enemy?
Screens, as written, do not "feel" like Star Trek in the slightest. Excepting a few TOS episodes where Scotty mentions a particular shield (the rear one *, can't recall if they numbered it) being down more than any other, all other times the shield rating of the ship is given as a percentage down. No direction, no weakening of any particular side, just a total shield strength percentage is given. Starmada's shield rating simulates that better than SFB's shield boxes, IMO.
* Rear could mean the back HALF of the ship, not 1/6 or 1/4.
Now, screens can represent a better regular degradation than shields in that shield damage has some affect with every shield hit.
Example: Hull: 10 ...
Screens: 18 17 15 13 11 9 8 6 4 2
Shields: 3 3 3 3 2 2 2 1 1 1.
It takes 5 shield hits to affect the Shield strength in this example, where as every shield hit affects Screen strength.
This means that the screens must be allocated as evenly as possible to equal shield strength in all directions, but that is costlier (not that it matters if every ship uses the same design criteria). But, what player will allocate screens evenly, or even at nearly the same ratio as regular SFB ships (i.e., front shield is stronger), instead of just allocating as much as possible to face the enemy without a rule forcing them to?
Now there are two things to consider: SFB-like, or Star Trek-like. If you want to attract SFB players, it has to play and feel somewhat like SFB. Since it seems that you will be restricted to the Star Fleet Universe, then SFB-like you might have to be.
IMO, SFB shielding is a poor representation of Star Trek shielding. Starmada can do better, it's just a matter of which flavor: shields or screens. I think shields fit better, but if some restrictions are applied to screens in how they can be allocated, then I would agree to screens being best (mostly for the damage effects), except for the extra plotting time it will take. Imagine plotting not only your moves but also your screen allocations for your 10-ship fleet. I'd rather not spend time allocating screens but rather get to moving ships and hearing the lamentations of my enemy's womenfolk.