Actually, when I bought the lens, I was initially directed to Canon's 50% off sale regarding the 55-25 non STM lens. Then I found the 70-300 also at 50% off. So, the 55-250 was $119 and the 70-300 was $259 (plus tax). I read reviews of both back and forth and the reviews tend to be mixed as to what is better, but in the end I opted for the 70-300 primarily because of FF compatibility.
Do you have a FF camera? If not, why does FF compatibility matter?
Is it because you MIGHT get FF eventually?
I see this opinion alot. People avoid EF-S lenses because they "might" go full frame one day. As a result, they are paying more for a lens that's heavier and bigger then it needs to be.
Lenses (ESPECIALLY Canon/Nikon lenses) simply don't depreciate in value very much (beyond the new-used transition).
Consider your case, you mention the 55-250 was $120, and the 70-300 was $260. Say you bought the 55-250 instead. Have you checked used prices for the 55-250? It's about $100-120. So, you could sell that lens today, and at worst be back $20-$30.
Yes, selling EF-S lenses when you buy a FF camera (if you ever do) might be a bit of a hassle, and if the future hassle is enough to warrant buying FF lenses so be it.
Personally, I don't see it. I buy the lens that's most appropriate for the body I have today. I don't buy something purely because I MIGHT get gear in the future that isn't compatible.
Note I'm NOT saying to AVOID FF glass. I owned the nifty 50 before I ever needed a full frame lens, mostly due to it's insane cheapest and it's wide aperture. If FF glass serves a need not available in the APS-C space (say a tilt shift lens) then by all mean go for a FF lens.