The best way for figuring it out is to use a website like edshipyard.com, where you can compare different types of SBs applied to a certain type of shield.
Most builds I use have about 50% thermal resist and 50% heavy duty, in order to maximize total effective shielding.
As stated above, explosive resistance is pretty much meaningless given how already good shields are against explosions... and how infrequently NPCs use them.
But FD does need to look into them. There's a nerf incoming eventually (they tried to nerf them twice already but backed down because of popular outcry) because they're waay too powerful. Somehow, FD devs had forgotten to consider what would happend if a player put all class A shield boosters with G5 upgrades on C8 prismatic shields... Even thought they should've known maximizing them would be the first thing every player would do... Now, since engineering is so g tedious, they can't really take them away because the mistake is purely on them (since those effects were very easy to predict at the development stage) and the community has probably spent thousands of hours engineering impenetrable shields...
The only way I could see FD fixing this would be to take a "take some, give some" approach where players are offered something else.
For an example, they could violently nerf the total shield points available by making SBs less efficient the more you equip them, but in return, make it so that the regeneration rate of a boosted shield is increased proportionally to the % of shielding the nerf took away, with the added difference that multiple SBs greatly increase the time before regeneration kicks in after a hit.
All those who would've just wasted hours upon hours of efforts won't have their efforts wasted since they might lose their "impenetrable" shield walls, but they get shields that regen much quicker, thus making PVP more thrilling and PVE as fun as before.
By the way ED: This wouldn't even be an issue if you hadn't made the whole engineering process so brutally not fun and unrewarding. *wink*