Buy the best quality you can afford
Based on my "professional experience" in optics for "other uses" , I would suggest buying the best quality you can afford as long as the magnification is 10x or more. I have looked at my razors and knives with my german 10x loupe and it shows very clearly the change between 2000 and 4000 and 4000 and 8000. I use other tests for sharpness after that but as far as seeing things I know quality buys you a better look than just going for simple magnification. Viet Cong snipers had 4x scopes and shot the hell out of our guys with an inferior round. German snipers also only had 4x fixed power scopes and were a formitable opponant. Thankfully quality american glass was available duriing both periods and today. My favoriate rifle has a fixed power scope made by an english company that hand laps and polishes their own optics, not sure who does their coating then they had fit each scope. 6x can be a dangerous thing!