If you look at history, you'll notice that things which are sold as collectibles actually lose appeal and value over time. The best collections are the ones no one ever thought would be worth anything. Coins are still worth collecting, but they don't gain value much better than anything else. However, you can bet they will always be worth at least their face value.
The best advice IMO is to collect what you love. If it becomes valuable, great. If not, at least you will enjoy your collection.