...there's no issue that it isn't a FFP.
...I wonder why the FFP scope uses 1/2MOA clicks instead of 1/4MOA. That makes a fair amount of difference at longer ranges (5 inches @ 1000 yards vs. 2.5 inches), and when you pay all that extra $$$ for FFP, you'd think they'd give you the finest adjustment possible.
Speed and versatility.
Staying at 18x works for you. It doesn't for everyone. I don't really care to debate the advantages of either focal plane - both obviously work, it just depends on your application. I've used both long enough to have sussed out the differences in my real world, and having an FFP works better for me. I have a few dozen SFP scopes, and only a handful of FFP's, I'd rather have all FFP's.
The disadvantage of a 1/2MOA adjustment is mostly a mental comfort issue, moreso than any practical implication. 1/4MOA is 2.5" at 1,000, 1/20mrad is 3.6", 1/2moa is 5" - pretty few folks can really tell the difference in a 2.5" shift on their groups at 1,000yrds, and those who can are able to hold over or under as needed just as effectively with any of the 3 above. I've wasted money on 1/8MOA scopes in the past, and quickly realized the difference is almost purely academic. I have to remind myself of that fact frequently, because it does SEEM like a disadvantage. Some guys claim 1/2MOA clicks are faster, and I'm sure they are for some folks, but I usually adjust looking at my graduations, not by counting clicks, so it's just as fast either way.
Regarding price: I (and you) get the FFP Viper HSLR cheaper than market price for the SFP version, and that particular model is about as cheap as it gets for the combination of features, magnification, glass quality, and FFP compared to other brands. So it's pretty easy to justify cost. I paid less for the 6-24x50 Viper HSLR's than I have for 4.5-14x50mm VX3i's SFP's (that gold ring is expensive!), less than the 4-16x44 and 6-24x50 Sig Tango4, less than the 4-14x50 Nightforce SHV F1, less than the 5-25x50 Burris XTR II, less than I paid for the 3.5-21x Bushnell Elite Tactical ERS FFP.... Let alone talking about the NF BEAST and ATACR, Tango6, Razor, S&B, Kahles, etc... I'm not aware of an FFP optic with magnification over 20x, or even 16x, which is cheaper than the Viper HSLR FFP - if there is one, I'd probably be shooting it instead for these few "less expensive" rigs.
My preference is for a scope with 1/10mrad, and frankly, I'd prefer have a scope which didn't say Vortex on the side at all, but for the feature set, the HSLR FFP has most of the features I want at a lower cost than other models. For budget builds, it has worked well enough, and better than most scopes in its price class. Glass isn't as good as the NF SHV or the Bushnell, about on par with the XTR - which tracks better than the Vortex - and on par with the Sig - which is illuminated - but for $100-400 less, I'd buy it again for another lower priced rig.