Which Would Be Better?

The "yellowing" is mostly due to the use of linseed oil in making the varnish, BLO will have much the same effect. I've never used tung oil, but have been told it will also darken over time, but not as much as BLO. Bear in mind that the wood itself will also change, generally light woods get darker, and dark woods get lighter. There is a varnish that doesn't darken nearly as much, it's Pratt and Lambert 38. P&L formulated it with soya oil instead of linseed oil. It's much lighter going on and stays much lighter over time. It's also extremely hard to find (at least around me), but it's my favorite varnish. But tung oil (like BLO) is very easy to renew, and should last just fine on something like that box. If it starts to fade in appearance, you just apply another coat, after cleaning the dust off of course.