300Ah deep cycle (50% DOD) versus 750Ah shallow cycle (10…20% DOD). Unmatchable.
You’ll have for real 150Ah from the deep cycle (that means approx 1.8kWh usable) at 500–600 usable cycles versus 75–150Ah from car batteries at no more than 200 such cycles. Car batteries will quickly be damaged, they are made with many thin plates to deliver a very high current in an instant, to be able to start the car, which absorb more than 1000A in the first half second and 150A after that until the engine starts.
The deep cycle batteries will serve you more than three years if used in this way (down to 50%) and maybe 6–8 years if used less.
If you consider a recommandation, I am using LiFePO4 batteries; they are a little more expensive (probably 50–100% more than your price for the lead acid you want, i only can tell that you can buy them for approximative 600$/ usable kWh, that means you have to pay 1200$ for the same usable capacity as lead-acid you want) but can be cycled 2000–3000 times to 80%DOD and probably more than 7000 cycles at 50% DOD (“probably” because these are only since 2005–2008 in the market).
They need BMS but these become cheaper these days, some 30-40 $ for the 4 cells LiFePO4 you need.