Most of the WP7 applications use either the Panorama or the Pivot controls. It seems that you can solve most of the UI requirements by just using these two controls. But, some care should be taken. The pivot control at a first glance may look like a perfect solution for apps where the content must be split across hundreds of small views (think book, where each page is presented by a pivot item).

But beware: the PivotControl is a memory hog. An app with 150 empty PivotItems uses over 55MBs of memory. After adding the pivot items, there’s not much of memory left to add the actual content, when the memory limit of 90MB is kept in mind.

PanoramaControl fares much better in a similar test: An app with 150 empty PanoramaItems use only 21MBs of memory.

Source:  http://mikaelkoskinen.net/post/wp7-performance-tip-beware-pivotitems.aspx