120fps or 240fps for slo-mo videos?

Personally I would look at 720 @ 240 FPS if you are a budget.

240fps is great but not at the expense of resolution. I've experimented with it a few times and while it's great for pellet tracking, that's all it's good for.

120fps still provides some great slowmo, 10x slow is still smooth depending upon which editing software you use. I've also used 16x slomo and had it turn out smooth.

Another thing to think about is lighting. 240fps will require good lighting, like a sunny day, for the best results. And still you'll only get standard HD resolution.

120fps does pretty well even on the cloudy days.

I have used GoPro Hero cameras for all of my scope camera work as they offer the highest resolutions and frames per second of any other action camera on the market. The only downside to the GoPro Hero's as the fixed lens focus which requires the user to manually adjust the from down from 11" to whatever the proper focus is for the reticle you currently using. For lens manipulation, the Hero 2, 3 and 4 requires a fair amount of plier grip and firm twist energy to break the epoxy they apply on the lens in the socket but once loose, the user can achieve any level of focus required. The Hero 5, 6 and 7 are far easier to adjust as I capture in one of my latest videos entitled "Tutorial: GoPro Hero 5, 6 or 7 Lens change", which demonstrates how focus or lens manipulation is achieved.

As for frames per second, it has been my experience in editing and producing hundreds of hours of high speed video that higher frame rates are great for capturing pellet in flight. a video shot at 720 @ 240FPS will certainly show more information but as Bob states, requires far better light. In general you will get much better results in better light. 

Below are examples of shots taken at 720/240FPS and 1080/120FPS, two will be basically in the same location with the same lighting and the third will be in direct sunlight. All these videos were captured using an older GoPro Hero 4 Black with a non fisheye 6mm lens through a Hawke Sidewinder 6-32 x 56 Scope.





https://www.youtube.com/watch?v=ZoaYge7VX2w

720/240 FPS




https://www.youtube.com/watch?v=KKW4tM4sukM
1080/120FPS





https://vimeo.com/278255734

1080/120FPS Direct sunlight

Now in the last 1080/120FPS you can actually freeze frame and see the shine of the shirt of the pellet just before hitting the chipper in the mouth. As you can see the clarity of the shot at 1080 even though it was shot at 120FPS with enough light, can capture the action.





Keep in mind the New Heros offer 1080 @ 240FPS now so you can have the best of both worlds.