It depends on what you are looking for/at. Sorry.
But some general rules. The higher the magnification, the smaller the field of view, so if you are looking for big game animals, more power might make you miss some completely that you could see well enough with lower power.
Higher magnification also magnifies tremor from your hands. Traditionally, more than 7x has been thought to mean most people need a support for the device for prolonged viewing or your eyes may tire quickly.
Higher magnification will also show up problems with lower quality optical glass that can tire your eyes and make it harder to see the details of the object you are examining, so if you can't afford the best quality, don't waste your money on really high powers of magnification.
The size of the objective lens limits how much light can enter the instrument, and the relationship between this and the magnification determines the exit pupil, which limits how bright the image can appear to you. Divide objective lens size by magnification to get the exit pupil. 7mm is about the biggest a typical young adult's pupil can open to in low light, so bigger than that will never be any use, and as you age you typically lose a couple of mm of pupil dilation, so if you are in your 40s, more than 5mm exit pupil might be a waste. Better quality glass and lens coatings can improve the brightness and image quality at any given combination of magnification and lens size, so make sure you compare like with like.
E.g. with those the 8x42 and 10x42 Bushnell EX Excursions you mention, the 8x42 will have an exit pupil 5.25mm, and the 10x42 which will have a 4.2mm exit pupil. The quality of glass and coatings should be identical, so the 8x will seem brighter and be better in low light, and have a larger field of view, and be less likely to tire my eyes, and not show the limits of the optical quality so much. Personally, I would choose those benefits over the extra 2x of magnification.
