Traditional MOLAP vs Tabular
Analysis Services now ships with two separate versions. The traditional version is the MOLAP version of Analysis Services. It is a true muilti-dimensional structure. The new tabular version is more of a relational hybrid that is based off of PowerPivot.
There is no migration path
There is no migration path between the two versions. They are two completely separate products. If you start to develop on one platform and then later discover that you need to switch, you will need to start over. There are no migration wizards. So it is very important that you choose wisely from the start. And while you are choosing you should also consider your future needs. It may be that the first cube you develop is fine for either platform but future cubes may have require one version over the other.
Differences between the versions
The two products are similar, but there are some pretty significant differences between the platforms. The traditional MOLAP version is a very mature product. (Over 15 years old) It is very feature rich and actually has more "features and functionality" than the new Tabular version. The tabular version is the new kid on the block and the future of BI for Microsoft. (That is, as long as they don't switch gears again, which has been a fairly common occurance.) The tabular version provides some query performance gains and in "most" cases it is easier to implement.