15-85 will be a good start, wider at the wide-end, longer on the long-end (comparing standard zooms)
it is a good compromise of the longer zoom-range, 18-200 is superzoom, superzooms usually compromise image quality.
With inter-changeable lens option, you will eventually be tempted to have more lens anyway.
most of us would like to have one or 2 lens to switch to, sooner or later, like a 70-300 (at least a 2 lens compo in the bag).
To me, I rather start with 15-85, plus 70-300.
it is better than starting with 18-200, subsequently buy additional lens:
- another standard zoom (17-50 or 17-85 or 15-85),
- Ultra wide zooms, (10-22, 12-24 etc)
- tele-zoom(lighter 70-300, heavier 70-200 F4 or F2.8, etc)
which is alot of duplication in zoom-range (if you already have 18-200, by then, you probably keep the 18-200 for travel or sell it away):dunno: