I want to buy some ND grad filters but am unsure whether to purchase the Cokin a-series or p-series. I prefer smaller and lighter equipment because it is easier to transport and since my lenses (18-55mm and 55-200mm) both have a 52mm filter thread, is there any disadvantage to buying into the a-series rather than the larger p-series?
The 18mm focal length of your wider lens would show the filter holder in the frame, if you used the A-series. Cokin themselves only recommend using the A series for 36mm focal lengths and above, but if you're using a cropped sensor camera you could probably get away with 24mm. Even the P-series are recommended for 28mm and longer, but again that would probably be more like 18mm on a cropped sensor, so you may just get away with it.
The P-series filters and holders aren't really too cumbersome, and even if you are restricted from using the wider end of your 18-55mm zoom, that would be less of an inconvenience than having to lug around and pay for the very expensive X-Pro or Z-Pro series.