ATPL. It's 14 papers as opposed to 7-8 or whatever, but it's nowhere near double the amount of work, as there is a lot of duplication across subjects.
Originally, all I wanted to get was an IR on my PPL but, after considering the alternatives the easiest option and best bang for the money was to end up with a so-called frozen ATPL.
I don't know how long you've been a PPL but since you are dedicated enough not to have given up after your first 100hrs and you're looking at expanding your horizons, chances are you may end up wanting to do an IR at some stage, both because of your improved choices when flying privately and because it will come in useful for your instructing.
Plus it's good fun.
Plus, as someone else said, if you're considering early retirement, you will have time on your hands and could very well end up being offered the opportunity to fly full-time, other than instructing (not at the moment, granted
). There is no point in restricting your choices for the sake of £300.-
Finally, the other reason I say this is because the two CPL-only instructors I met ended up regretting their original choice. In fact, one of them I met at Bristol where he was preparing to sit his 14 ATPLs.
[Sorry, I appear to have repeated most of Orbit's points above. Should have read his/her post better]