Found it. Edited it to answer your specific questions:
Arabs are great for bonding. They're intelligent, yes, and you have to know how to cater to their intelligence. You can't bore them, but you can't ride the same pattern every day in the same order with the same routines....even if it is interesting. They'll start anticipating your next move. They'll jump into a trot as soon as you shorten your reins. They'll change leads early when doing figure-eights if you do them constantly, because they're incredibly smart and know what to expect--they memorize.
My uncle and aunt have an arab and half-arab reining ranch in Colorado I love to visit, and their horses are NOT high-spirited, out-smarting little devils because they know how to work with arabs. They change things up and put their intelligence to work in a good way, where both horse and rider are comfortable.
I rode all of their arabs when I was there, fell in love with one of their horses (National Show Horse [arab x saddlebred cross] in my avatar, worth $30,000) and man, was she smart. I taught her to bow in thirty minutes one day. I still can't get my Paso Fino to bow, and I've been working on that with him for a year! I spent one day of riding her just working on "throwing her out" with the reins, then gathering her in, working on not letting her change pace, even when she expected to go ahead and trot because horses often associate gathering the reins with going faster. At the end of the lesson, she understood perfectly that she was not to change pace unless asked.
Thoroughbreds....Well, I have two, and let's just say they need ridden every day. If you can give that to them, they'll be happy. They're probably a better choice if you're looking at their competetive, jumping and dressage aspects alone, rather than temperament. Personally, I prefer the NSH or arab to the TB for temperament, but I honestly would never go out and buy an arab for eventing. If you're looking at endurance riding, arabs and tb's are both great, but you see more arabs in endurance than tb's.
Looking at calmness, when handled correctly, arabs are a lot less hyper and disrespectful than most tb's. Thoroughbreds are bred to bring out the "flight" in the "fight or flight" aspect of the equine mind, and therefore have a tendancy to bolt. As the owner of two off-track-thoroughbreds, I can tell you neither of them are suited for inexperienced riders at this stage (they're ages 5 and 6), but there are the few TB's that are good beginners' horses. If you're just learning to ride (and again, I don't know your experience level), I would not reccomend most Thoroughbreds. Maybe start out by taking lessons on or leasing one of either breed to see if you like their temperament. And again, every horse is different, so don't pass on one breed just because of an individual horse.
Thoroughbreds are so cheap because off-trackers are produced like crazy. They're run, few are kept, and the rest are sold for dirt cheap just to get rid of them.