Depth of discharge will be relevant in your case. A lead acid battery should only be discharged to no less than 50% of it's capacity if you expect it to last for a number of years. If you are pulling it completely empty until it stops working in 4-5 hrs (damaging the battery) then as already said there is not much gain in going Lithium. If you stop use at 50% on the lead acid set up then you will get longer run times from the Lithiums as they can be used completely with no negative effects.

Chris