I currently have 4 combat ships and 1 hybrid trade/combat: A Chieftain, a Krait Mk II, an FDL and a Corvette for combat and a Cutter is the hybrid.
Ignoring the Cutter and Vette, the FDL is hands down a better PvE combat ship. Stronger shields, a good set of hardpoints for your weapon preferences, and decent speed and handling combined give it just a bit more "oomph" than the others.
That being said though, for PvE bounty hunting all of them do a great job and I think you'd be fine picking whichever of the two ships you enjoy flying the most. If you enjoy flying both though, go with the FDL so you don't second guess yourself later.
As
@Bigmaec has already said, you'll want it engineered fully before tackling Thargoids.
For build, I recommend you make weapon choices based on your own preferences and combat skills rather than an off-the-shelf build, this is the one I use as someone who can fly pretty well but not well enough to aim fixed weapons particularly effectively. I do all PvE combat activities except Thargoids in it.
A ship builder, outfitting and comparison tool for Elite Dangerous
coriolis.io
The guardian shield boosters were Shield Cell banks in my build before I unlocked the guardian tech and I still keep the SCBs around to swap to once in a while. I just generally prefer passive defense over active. The Sensors and Life Support in my build are A rated and quite arguably would be fine as D-rated. I just like the longer range sensors and the security of a larger air supply (I've never needed it in the FDL though).