First of all, I know philosophy doesn't offer a 'secure future' in that it isn't obvious what you can do with it, but it's definitely employable. Remember something like 40% (or is it 60?) jobs don't specify what subject the degree should be in. Also, say you wanted to be a lawyer, you could get a degree in philosophy and then do a conversion course. I wouldn't personally say SPS is that more employable anyway- they're pretty comparable.
With the tripos thing, you could spend one year doing philosophy and then switch to SPS, or vice versa. Or you could spend two years then switch. I *think* (though someone can clarify this) you would graduate with a BA in whichever subject you did Part II in, but obviously you could mention to your employer that you have Part 1A and/or 1B in another subject.
Don't choose based on employability- let's face it, Cambridge is a good university so regardless of your subjects, I'm sure you'll get a job. If you prefer Philosophy, go for it-- you're more likely to get in anyway if you're genuinely enthusiastic about it.
I'm a first year philosopher by the way.
Edit: Oh, and another option would be to abandon the idea of studying at Cambridge and go for Philosophy combined with Politics elsewhere...I don't know much about that but there are a variety of courses.