Yes there is a difference.

What do you need them for? I find I use my metric more often as most of the stuff I play with is Japanese or Swedish. Most American OPE is standard unless the engine is from overseas. U.S. autos use a mixture, I needed both to remove the transmission from an 85 GMC.

Like canguy said you need both unless working only on American or only foriegn made equipment. 5/16 and 8mm cross over fine but the rest really should be specific.

You need both to cover all bases. As a mechanic I started out with little money and made things work with whatever I had. I had no choice. I learned to use many metrics sizes on standard and visa versa. Beyond the 5/16 to 8mm crossover there are many others. The most common being the folowing. 7/16 fits 11mm, 1/2 fits 13mm., 9/16 fits 14mm, 5/8 is close to 15mm, 11/16 fits 17mm, 3/4 fits 19mm. There is no crossover for 10mm or 12mm. So .....yes you need both sets and there will be sockets in there you probably will never use and if you are like me you may lose a few of the most common sizes.

I boxed up all my metric tools in 1994 and haven't touched them till this past fall. Bought a John Deere bagger for my LT and every stinking nut and bolt that came with it was metric. I was not a happy camper.

If you drive traditional American made cars: Ford Chevy..., then you'll need Standard, if you buy foreign, even those made in America: Toyota, Mazda... you'll need metric. I have both types of vehicles. I'd say most of the time I use Standard and when I buy hardware: nuts and bolts, I buy standard.
I have both and can't imagine trying to live with the "cross-over" use and an adjustable wrench for the rest, but it can be done. If you are real tight on money and you don't expect to use one much, but a cheap set of that set, say from Harbor Freight. I regularly purchase form HF and find their Chinese stuff good for the money. Even most American brands are made in China these days, ans so state, but hey may be held to a higher specification for quality than the off-the-shelf Chinese stuff one gets from HF.

I will buy a set of each. Thank you to everyone. You have been great help. I don't mind getting my fingers dirty.

jerry_nj , Not being argumentative but Fords and Chevys are metric. Have been for years.

Thanks, I have a 2005 Chevy Colorado, and the good news is it hasn't required a wrench, beyond a strap wrench for the oil filter, since purchased new. I did not know, and am not surprised given the global marketplace, that US auto mfgs changed to the world's standard, metric. This makes the importance of having a metric set of tools more important.