Wet lead acid batteries have approx twice the internal resistance of AGM batteries, hence AGMs have half the internal voltage drop under load.
The higher the current, the higher the voltage drop.
EG - if internal resistance (ESR at full charge) is 3mR (0.007 Ohm) and cranking current is 250A, then the battery voltage is 12.7V (full charge) - IR = 12.7 -(250x.007) - 12.7 - 1.75 = 10.85V.
For a wet cell it would be 12.7 - ( 250 x .014 ) = 12.7- 3.5 = 9.2V.
The less charged the battery is, the higher its internal resistance, hence a larger voltage drop for same current. (From a lower internal voltage, but that is similar for both wet & AGM batteries - ie, 12.7V full; 11.3V fully discharged.)
In the above example for a 250A starter motor, we compared a 1.75V difference due to battery alone.
But cables can also make a big difference - hence the "Big 3" - especially for the starter the battery +12V to starter, and the battery -ve to engine ground.
For other loads, the battery +12V to the load, and the battery -ve to the chassis/body to the load (ground).
Hence why PC (and audio) supplies are often dedicated add-ons that avoid voltage drops though "least cost" cabling, ignition switches, various fuses and connectors.
Fuses add resistance (especially near rated capacity) - far higher than the cable they are protecting.
Why mention the above?
Because voltage dips at PC PSUs can be overcome by good wiring (the Big 3, and dedicated cable via a fuse and perhaps a relay)..
Or a second battery for the PSU (which is only connected when the vehicle is charging; hence ensuring independence from the main battery in case the PC etc drains its battery when not charging).
That may overcome the need for expensive AGM batteries as cranking batteries. Deep cycle batteries are not designed for cranking. And whilst some say AGMs are good for both deep cycle and cranking, what they really mean is "all AGMs provide good cranking current" by virtue of their lower internal resistance - it doesn't mean they like doing it. (How often do you see CCA etc for AGM batteries? Not often? They usually provide short circuit current or internal resistance instead don't they? Certainly if they are deep-cycle or slow-rate alarm etc AGMs.)
If you can only have one battery, then fine - AGM.
But I prefer a standard wet cell main/cranker, and then whatever other battery I want for the application - whether deep cycle; a small AGM in lieu of an inferior stiffening cap; or a big AGM for thumping audio or winches. (With automatic isolator of course!)