What GPU should I choose for my old CPU?

This is not really all that accurate. Both graphics cards offer a relatively similar level of performance, with the 1050 Ti being only a little faster, so any "bottlenecking" would be fairly similar for either card. If the prices of the two are about the same where you are, I would go for the 1050 Ti, as it is not only a bit faster on average, but also has double the VRAM, which could help performance in some newer games. It also requires significantly less power under load compared to a 960, and in turn should put out less heat. Another option would be a regular 1050, which is only slightly slower than a 960, and has 2GB of VRAM, but might potentially cost less.

As for "bottlenecking", the term only refers to one piece of hardware limiting the maximum performance of another. In your case, there will likely be some games that utilize all the available performance of one or more of your CPU's cores, and the graphics card will need to wait for the CPU to complete its processing each frame. Aside from holding back your frame rates in the game, this shouldn't really be a problem. If the CPU can only process the game's routines at a rate of 40fps at a particular point, it won't matter whether you have a 1050 or a 1080, either way you're not going to get more than 40fps at that point. So, you certainly wouldn't want to spend a ton on high-end card, but cards at this performance level probably won't be limited too badly. If the CPU is limiting performance in a particular game, you might not be able to get higher frame rates, but you can always put that underutilized graphics card performance toward turning up the graphics options for better visuals without affecting the frame rate too much.

Edit: One other thing to note though, is that if you have an older monitor that requires an analog VGA cable connection, those are no longer natively supported by the 10-series cards, and you would need an active adapter to hook one up, which can be found for around $10 on sites like Amazon. If your monitor supports a digital connection like HDMI, DVI-D or DisplayPort, that shouldn't be a problem though.