Shields depend on your mode.
If you play in open, I strongly advise to go reinforced, because otherwise some plasma armed ganker will have easy prey.
For PvE thermal is "ok", but you still get better numbers depending on your utility slots if you go reinforced and add a thermal resistant booster.
Fdl example following
first one with reinforced shield + 3 HD Booster 1 Res aug 1 thermal
A ship builder, outfitting and comparison tool for Elite Dangerous
s.orbis.zone
second one with thermal shield + 4 HD Booster + 1 Res aug
A ship builder, outfitting and comparison tool for Elite Dangerous
s.orbis.zone
The reinforced shielded FdL has ~2200 absolute mj, ~6000 mj explosive, ~5000 mj kinetic and ~3300 mj thermal
It beats the thermal shielded FdL by ~400 mj absolute, ~1700 mj explosive, ~1900 mj kinetic and still ~300 mj in thermal
This difference will be less by having less (<4) boosters, and greater by having more (8) boosters.
Another caveat regarding thermal is that in my opinion thermal g4 is better then g5 because it gives a better balance.