To answer DeltaT's original question: it's worth continuing instructing after 1500 hours, but you need to be aware of a few traps.
Cessnababe and others have already ripped you to shreds about complacency. Personally, I think that is a little unfair- the whole PPL industry operates on the basis of instructors being prepared to accept a low salary, for a finite period before moving on. As an FIC instructor, cessnababe is part of this system- if instructing as a method of building time towards the ATPL came to an end, I think she'd find she had fewer FIC students...
A related trap is the "bad habits" problem. My solution was to keep adding extra qualifications: I instructed for almost 5 years, during which time I went from basic PPL instruction, through Flying Scholarship work, IMC and Night Ratings, an informal standardisation role (essentially in soloing other instructors' students), into CAP509 instruction. The pressure to meet new targets forced me to keep improving my own skills, which helped me career-wise, but also helped my students.
Sure, you don't need to do these etra ratings to be a professional instructor or pilot- but they do serve as proof to an airline interviewer that you took the job seriously, and are likely to do the same once you join them.
Take the commercial instructing opportunity- you'll earn more money, you'll feel more involved, the extra responsibility will help your personal development,
and it'll look good on the cv!