I think you both have reasonable points.
At the end of the day, the truck selection is pretty arbitrary in terms of balance. Some of us were of course setting ourselves our own challenges playing in standard mode before this, but Hard Mode should ideally be a challenge that applies restrictions such that you have to exploit every tool available to best effect to get through it. Clearly with the current truck selection, that's going to mean either gravitating towards a select few trucks, or carring on as before, and arbitrarily introducing our own personal-honour-system to the mix to keep things diverse - so we're back to square one, not fighting tooth-and-nail to beat a tough challenge, but intentionally holding back to make things interesting. Hard Mode should be the solution to that, not a continuation of it.
I don't think basing XP on the relative balance-of-power of each truck would work as a system - XP is a limiting factor early on, but soon enough you hit Level 30 and unlock absolutely everything anyway... then it would be meaningless.
I can't really think of any solution that would work besides doing a bit of truck rebalancing. The biggest anomaly seems to be the Azov 64131, which just has a preposterous list of positives, and negatives that total "turning circle a bit poor".
As Hard Mode is very much focused around fuel consumption, I think the overall fuel consumption delta between trucks could stand to be stretched out and rebalanced. The "always on" trucks (mostly the Russian ones) with the "special"-type gearboxes just don't fit the scale currently, as those special gearboxes are specifically set up to not add extra fuel consumption for having AWD turned on. That really could do with being addressed. In Hard Mode, having the benefit of "always on" AWD should really be compensated with worse fuel economy, not better.