It seems to me that it is better to have a larger hull without Armor Plating than the smaller hull with it.  For the same CRAT, the hull can be 50% larger.  Here are ship examples:

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(290) Armadillo-class CA

Hull: 10 9 8 7 6 5 4 3 2 1             
Engines: 4 4 4 3 3 2 2 2 1 1             
Shields: 3 3 3 3 2 2 2 1 1 1             
Weapons:
1:X 2:X 3:Y 4:Y 5: 6:

X: Blaster: 6/12/18, 1/2+/2/2
Piercing
[AB][ABC][ABD]

Y: Autocannon: 1/2/3, 3/3+/1/1
[ABCDEF][ABCDEF]

Special: Hyperdrive; Marines (6); Armor Plating; Anti-Fighter Batteries (2); Cargo ( 8 )

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(290) Horned Toad-class BC

Hull: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1         
Engines: 4 4 4 4 3 3 3 3 2 2 2 2 1 1 1         
Shields: 3 3 3 3 3 2 2 2 2 2 1 1 1 1 1         
Weapons:
1:X 2:X 3:Y 4: 5: 6:

X: Blaster: 6/12/18, 1/2+/2/2
Piercing
[AB][ABC][ABD]

Y: Autocannon: 1/2/3, 3/3+/1/1
[ABCDEF][ABCDEF]

Special: Hyperdrive; Marines (6); Anti-Fighter Batteries (2); Cargo (704)

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The only combat difference is that the BC does not have Armor Plating, is 5 Hull larger, and there is one less chance to hit a weapon on the weapon damage chart.  The larger hull has more hull hits, engine hits, and shield hits.  It also benefits in more sustainable repair facilities (it takes 7 hits to reduce the repair crew instead of 2 hits for the CA).

For the same cost, and the BC could be cheaper by reducing the hull a couple points (13 hull, CRAT=272), I'd rather take the BC.

(P.S. I modified my copy of the shipbuilder to automatically list Cargo with all the space left over)