A 17 xtel is about £100 a 28 ft xtel hybrid about £300, both great poles. A SLX is £400 and is an unbelievable pole, as light as the 17ft but almost as long as the 28ft xtel, and a lot more rigid.
If you have no 3 story work yet then just get the 17ft, and buy what you need when you need it at a later date otherwise get the SLX. The 28ft hybrid is still a good pole though.
Simon.