If you run the numbers:

Using 1.5 qts/lb as your water to grain ratio and assuming that your maximum grain weight is 18 lbs.

1.5 qts/lb * 18 lb = 27 qts or 6.75 gal of strike water

Grain displacement = 15 lb * ~0.32 qts/lb = 4.8 qts or 1.2 gal

As you can see, this scenario would absolutely max out the 30 L cooler and make it hard to stir. Your maximum gravity using 18 lb and a 75% efficiency would be:

37 ppg * 75% efficiency = ~29 ppg

(29 ppg * 18 lb)/5 gal = 1.103 S.G.

If you crunch the same numbers with the 40 L cooler and an increased grain maximum you get:

Using 1.5 qts/lb as your water to grain ratio and assuming that your maximum grain weight is 20 lbs.

1.5 qts/lb * 20 lb = 30 qts or 7.5 gal of strike water

Grain displacement = 20 lb * ~0.32 qts/lb = 6.4 qts or 1.6 gal

Your maximum gravity using 20 lb and a 75% efficiency would be:

37 ppg * 75% efficiency = ~29 ppg

(29 ppg * 20 lb)/5 gal = 1.160 S.G.

If you do not plan on making humongous beers or you plan on brewing < 5 gal, you can most certainly use the 7.9 gal cooler. If you mash in stiffer than the 1.5 qts/lb I used you will have plenty of room to make 5 gallon batches of humongous beer.

If you consistently brew big beers or are going to be consistently making 5 gal batches then the larger cooler may be a better idea as it grants you a bit more leeway to change the water togrist ratio or increase the size of the grain bill.