The are both good guns. You should be happy with either.
In terms of whether you can load them, you just adjust the air pressure in the gun to what suits you. So, if it is too tough to load in the beginning, you take a bit of air out. And then as you get used to loading it, develop better technique ,you can add some air (perhaps 50-100 pump strokes at a time). It is easier than it sounds and the same for both guns.
Gazz here shoots the Predathor 115 (130?) in Australia and takes some big fish, perhaps search for his user name and read some of his posts. Here is a spearo in Panama taking big snappers and the likes in slightly murky waters with his Evo 100:
BTW, Marco who is the shooter in the video says his Salvi 115 (he has one of those, too) is slightly nose heavy but nothing too uncomfortable. But I know a spearo who even added a tiny piece of cork to his Evo 100. These things can be quite personal.
Don't get too hooked up on videos, both guns shoot great. Theoretically, the Vuoto should have a bit more power as it is a dry muzzle gun. But if you want that, you can upgrade the Evo later on. It would cost about 40 euros for the cheapest option and 65 euros for a beautiful custom one from Tomba (search the forums here for Tomba).
Both will shoot a 7mm spear, both will shoot fast. But perhaps the Predathor might shoot slightly faster on account of the vacuum/dry muzzle. That muzzle has a seal in it that will wear out and need replacement once in a while. It is easy to change, you don't need to depressurize the gun and it can be done in a minute. But if you end up getting the Predathor, get some extra muzzle seals when you order the gun (about 9 euros for a pack of three).
Some say the Salvi has a harder trigger pull than the Evo, but the trigger pull would still be lower than guns like the Asso and Cressi SL and spearos have taken fish on those guns for decades.
The Predathor will ship with a reel in sizes over 85, so that should be added value. Personally, I am still undecided on the quality of the reel (I haven't shot my Predathor yet, but perhaps Gazz can weigh in).
The Salvimar gun used to ship with a line slider that was allgedly very brittle and could snap too easily, but newer ones are supposedly of better quality. The Mares gun have an issue where some spearos have broken the line release. Supposedly, Mares is a aware of this and might update the design but when and if it will get into the market, I don't know. Neither is a deal killer for me, actually but it means, you might have to tinker a little with them yourself (like with most spearguns).
To continue on in the tinkering department if you have such inclinations: The Salvimar handle is pretty good for my hand, though a little big. Certainly bigger than Sten handles. (I wear size M gloves). But I suspect you could sand it down a bit if needed as the wall thickness is quite high. I am pretty sure both handles come as ambidextrous handles from the factory, maybe the Mares one has rubber inserts to make it a righty or lefty. The Salvi doesn't. You can always make a custom handle later on. There are many tutorials on how to do that and it is quite easy. Again, not needed.
At the end of the day, it is one of those things where it is very hard to offer good advice as they are both good guns. Both will shoot great, better than most similar sized or longer band guns and both will get you fish.
If you feel like saving a bit of money, the Predathor is often nicely priced on scubastore.com. If you are lucky enough to have dive shops near you and you can see these in person, then Pete is right. Go check them out. But I suspect you don't have that
[EDIT]
I will add some measurements I took of my Predathor 85, as Salvi guns are a little longer than most other airguns (A Salvi gun is about 7cm longer than its model length, but if I am not mistaken, the Mares guns are the length they say they are in the model name).
Add 15cm to these measurements and you will know how big a Predathor 100 is:
PREDATHOR VUOTO 85 MEASUREMENTS
Gun Length: 924mm (muzzle to upper end of handle),
Gun Length loaded: 1103mm (to upper end of handle)
Original spear: 97cm (79cm inside barrel)
Overhang: 180mm (stock spear)
Height of handle: app. 134mm long (to rubber plug), app. 150mm tall