Don't worry about durability from scratches. It doesn't matter what material you use -- if a rock is gonna hit something, it's gonna leave a mark. Cast vs. calendered? Yeah, the calendared is thicker, but that doesn't mean "better" or "more resilient". we're only talking about the difference of a few mil thickness. That's nothing. Cast will hold up better over time (less lifting / curling), and it's the correct material for EVERY vehicle job. Calendared films aren't designed for long term wear (many times having permanent adhesives) and they're a PITA to remove. More likely to damage paint, etc.
SLX is fine for a "budget" cast film (is anything 'budget" these days? lol), but for vehicles I care about (hint: all of them), I use 3M IJ180 or IJ175 + 8518 gloss lam. I've also had good success with Avery. I'm sure Orafol/Oracal is good too -- many folks love Orafol around here. I just personally haven't tried them because my regular distributors don't carry it. For me it's 3M all the way.