get both! 
If it helps, the difference is about two pounds.
These weights are a great deal in my book. I can get two of each 8 and 10 lb for the same price I paid for 2 six pound weights off Amazon. Of course, that was a different brand, but a weight is a weight as far as I’m concerned, so I jumped on this deal
The 8 lb weight is listed at $19.42 (each) on Amazon. Most 8 lb weights out there like this one are at least $10 or more, plus shipping, which is generally considerable, considering the weight (no pun intended). This is a bargain. I was looking for some weights in this range…looks like I got lucky!
A gallon of milk is ~ 8lbs. Will you want more or less weight than that? Or is that just the right amount?