Assuming the OP is correct about the two engines being ISL's, then discussion of ISB's and ISC's isn't relevant.
All MH ISL's develop the same peak torque at the same RPM, however the higher rated ISL's will make more torque & HP, but ONLY when at higher RPM. Torque is identical from 1200 to 1600 RPM. At 1800 RPM the torque advantage is measurable on a dyno but not detectable on the road. At 2000 RPM the torque advantage is only 25 ft lbs (2.4%). The torque advantage at 2200 RPM finally becomes noticeable because the torque of the lower rated engine falls off sharply at high RPM. Since HP is a function of RPM, knowing the torque at a given RPM is all that is needed to determine both HP and the comparative ability of two different engines.
The result is that in routine driving there will be NO difference at all. If you are one of those that simply floors the pedal to accelerate or get to the top of the hill, then you will get there a bit faster but only when the engine is in the 2000-2200 RPM range. Also, while at those higher RPM you will be burning more fuel. Not just because it takes more fuel to get more HP out of the same engine, but because diesels burn more fuel when making the same HP at higher RPM than at lower RPM.
So, if you are like me and keep the engine RPM down to conserve fuel, then you won't notice a bit difference because you'll be cruising at about 1600 RPM and going up most grades at 1800 RPM or less.
Then there is the bragging rights in the camp ground advantage of having 400 HP. If that's an issue--and it is with a lot of people--then just stick a '400 HP' plaque on the rear of the coach and no one will know any better. The cheapest HP kit on the market.
OTOH, if the 350 HP ISL was really an ISC or even a new ISB-XT, then you're talking apples and oranges, because the smaller engines have less torque across the RPM band.