Basically what MechEngg said. The hydraulic tensioner is a hydraulic unit that, over time, loses its hydraulic "power" and lets the belt slack up, causing it to prematurely skip teeth and in turn smash valves into your pistons. The manual tensioner uses an old-school 16V design (I believe) where a manual lobe is turned to apply proper tensioning and is then torqued in place to hold at that angle.
The IE kit, plus the manual tensioner tool, is a little more than the "all-in-one" kits from ECS or MJM... but you'll never have a weak hydraulic tensioner and it will be better-than-OEM parts. Assuming you do your due diligence of checking the belt tension, which is easy, you'll have a trouble-free timing belt experience.
IE kit w/ pump (06A block): http://www.intengineering.com/Integr...27074-1-2.html
Tensioner tool: http://www.intengineering.com/Mechan...68569-1-2.html
You'll also want to pick up the motor mount bolts for the passenger side.... altogether the manual kit, done right, comes out to like $320 - $340 depending on whether you get the motor mount bolts locally or shipped from an online store.
In the end, it's up to you on what you want to do. The OEM tensioner works good for a lot of people, but some people have had bad failures. If we replaced every piece that has ever, even if only once, failed terribly... we'd have replaced every part in our cars by now. You should be fine with either kit... it just depends on how much you want to spend to do this job and what you think your car is worth. Whatever the choice, replacing the belt is a good thing.