I would always say that it depends on both the water and air temperature of where you paddle and to some extent how much you swim whilst doing what you love and how much cash you have to spare.
I think the American is pretty warm in the summer but can be a bit cooler in the Spring? If you end up swimming a lot as a beginner then a dry top won't help that much keeping you warm all day.
Over the last 25 years I've tried just about everything and the only way you really stay warm and dry is watching kayaking videos at home. However as it sounds like you are starting out and don't have so much cash here is my advice:
If you already have a pretty good spray top then a longjohn wetsuit would keep you warmer and still allow you pretty good freedom of movement. NRS makes good stuff and I would go for the Ultra john wetsuit for 20 U$ extra as the plush neoprene is much nicer. For sure you want to try one on in a shop before you buy it, as a wetsuit that is not tight fitting will be only slightly warmer than a paper bag. The other plus for a wetsuit is that it does give you a bit of paddling if you take a swim on a rocky river. If you go to paddle on a warm river on a hot summers day then you don't have to wear it.
A quality wetsuit will last you for years and you don't have to repair it much. A drysuit is much better for colder rivers and colder air temps, however even the cheep Palm suit will have to have the seals replaced from time to time and on a dry suit that's 5 seals! You also have to be careful with storage and the zipper. If the rivers are not cold the whole year round it's not such a good choice unless you boat the winter through.
A quality dry top is worth the expense but will tend to leak a bit with time. It will also have to have the seals replaced from time to time, mostly the neck seal. As I boat on fairly cool rivers I will wear a pair of NRS Hydroskin pants with boarder shorts over the top and a fleece or two under the drytop most days. For rafting I generally wear a 3mm wetsuit and a normal cheep spray top with a fleece under it.
Hope this helps,
Neil Newton Taylor
www.swiftwaterrescue.at