re: Meopta Meostar R1 3-10x50 vs. Leupold 3.5–10x40mm

Both the Meopta and the Leupold are excellent brands, so you're on the right track. I would consider the MeoPro and the VX-3 along the same line in quality, and the MeoStar and the VX-6 together. Performance wise and clarity I don't think you can go wrong with any of them, but you may be a bit more impressed as you shell out more money.

To answer some of your specific questions:
You will get better low light performance at higher magnification with the Meopta due to its 50mm objective, than you will with the 40mm in the Leupold, due to the optics exit pupil which allows a certain amount of light transmission as your pupils dilate under low light.

Here is where you need to make up your mind on what you want out of the gun. A smaller objective allows you to mount the scope closer to the barrel increasing accuracy at long distances, it is also more comfortable to look through (I have a 3-10x50 MeoPro and I hate how high it is). While a larger objective will allow you to see clearer later at the expense of weight and previously mentioned disadvantages.

100-300yds is in the range where spending good money on a scope is helpful, but you can accomplish what you want without breaking the bank. You may be able to tell but I'm a bit of an optics nerd so I would buy a VX-6 2-12x42