The Harris poles used to be good, but the quality went down hill a few years ago and I used to find I was getting through one every 6 weeks due to the clamps slipping.
They can be bodged up, but for £15 I just used to buy a new replacement.
I've never used a CLX but im sure if you buy one you will want to move on to an SLX at a later date.
My advice would be to buy the best you can afford and it will be cheaper in the long term to wait a while and save some more money if you need to, and buy the SLX.