Jet, either can be used to accomplish what you want. I am speaking of the Hayward models that I have LOTS of familiarity with, the CL 200's are the in line, the off line are CL220. The off line are easier to 'uninstall' when you don't need them - all you have to do is unhook 1 of the hoses from the unit and connect it to the fitting on the other (influent/ effluent) fitting and the chlorinator is no longer attached to the pool's plumbing. You can do the same thing with a CL200, in line, unit, but you'd need to get an extra SP- 1500-UNPAK set and the matching male union ends, so that you coul'd take the in line unit out when it wasn't needed. The CL 220 units have a bunch of small (and ~ inexpensive) parts that need replacing every year or 2, unless you take care of them.
With either scenario, I would remove any tri-chlor pucks and rinse it out whenever it was removed -- the pucks can be stored for the next time you need to reattach it.
If you need more info, simply ask! :-D
