I've used Amsoil for years in many vehicles, long before ever becoming a Dealer, and have been happy. My Dad even before me back in the 70s soon after they first came out. Of the two choices you mentioned though, Mobil 1 would be my choice over Castrol Syntec. Syntec is a Group III oil which is made from petroleum but the API allows to be called synthetic. It goes through a different refining process that makes it better than the standard Group II petroleum oils, but it still in the long run has the same characteristics due to the limitations of molecules that make it up. Most Group III synthetic oils use a base stock that barely makes it into the Group III range that the API says they need to be in. The higher the quality of the Group III base stock, the more it costs the manufacturer selling the oil. In many cases, they aren't much better than a good Group II oil, but you'll pay more because it says "synthetic" on the bottle. A Group IV oil is PAO based. This is what most Amsoil and Mobil 1 products are still made from, and used to be the norm until Castrol changed to Group III and the API allowed it to be called synthetic back around 99.
As for extended drains, Mobil 1 has had an extended drain oil out for a little over a year now, but this isn't something new. Amsoil was the company to first use the phrase "Extended Drain" back in the early 70s when they came out. One thing to note with the new Mobil 1, their warranty for this milage is only on vehicles outside of the manufacturer warranty. Until then, they recommend normal drains. True most oils can run beyond the normal drain interval, however, should something go wrong, then you are on your own. I'd at least use oil analysis to determine how the oil is holding up in your vehicle, unless the oil company says to run it that distance, then you can feel more safe, since you know you are under their warranty.
If you're looking for the extended drains for highway driving, yes, synthetics, in particular one offering extended drains, will save in the long run. However short drives are harder on the oil than highway driving. Each time you start and stop your engine just to run to the grocery store two miles down the road, your oil is going through a cycle. In many cases, the engine might not even reach normal operating temperature, which can be even harder on the oil. In cases like this, a good synthetic would also be a good choice because it remains more stable and typically the extended drain ones have more additives to handle the stress, since they need them to last longer to begin with. This is why Mobil 1 EP promotes drain intervals of 15,000 miles or one year and Amsoil 25,000 miles or one year. You might only drive 5,000 miles, but if they are the result of a few hundred short trips the oil could be just as worn as someone who drive 20,000 miles that year with only 50 startups. Also consider where you live. Many states, like here in Florida, are considered Severe Service due to the heat. This beats up on an oil as well. In many cases this even lowers the drain intervals on those oils offering it.
I also agree with sgarlic, a good filter is a must have, especially for extended drains. If a poor quality filter is used, with any oil, all you might be doing is circulating the the contaminants and particles.