Multiple projects - similar platforms or as different as possible?

Question

When working on multiple projects simultaneously (for the sake of simplicity let's say half time each on two projects), which is better? Should the two projects

• Use the same language? Same/similar frameworks?
• Use entirely different languages?

Additionally, is it best if they are target for similar platforms, or different (web v. desktop v. mobile v. utility library for internal use)?

The first responses that come to mind are

• They should be similar, and play to the strengths of the developer to help him handle managing both of them
• They should be as different as possible to help the developer keep organized

Since I've never done this in a professional setting, I was hoping more experience programmers could shed some light on the best way to go about doing this.

EDIT:
This question is assuming the existence of multiple other projects, each of which is underway (I think some underway and some brand new with no design decisions yet made would be too broad for one question). When choosing between existing projects for a second project, should a developer consider how similar his new project is to his current project, and how should that affect the decision?