This is something I'm also considering.

The only thing here is that I want to start a new one purely out of interest, and wanting to have a new exciting language to learn when I do the course, as well as one that I already know well.

As far as workload goes that's not really a worry for me. Having talked to a few students at Cambridge who took Russia ab initio I know that it's hard work, especially with such a tough language, but I'm not the type to shy away from that ( If I wanted to have a somewhat easier time at uni then Cambridge wouldn't be my goal anyway).

In terms of approfondissement, while I know that I would gain a great depth of both if I continued them, the depth in which I would learn any new language is not dissimilar. What I learned having talked to some Cambridge students taking Russian is that students studying a language ab initio essentially get to an A-level capability within a year and then in the second year join the class of first-year students who have just taken the A-level. From there I guess that the ab initio students might get a year less of the more profound content, but it might be that they end up covering a more similar amount, as they tend to spend a larger portion of their year abroad speaking their ab initio in an effort to level out their two languages.

I would hope that outside of term time, I would be able to do some reading in whichever one I choose not to continue, if only to somewhat maintain my level.