The Core i5 processor in the entry-level 15"
MacBook Pro model is significantly faster than the Core 2 Duo in the 13"
MacBook Pro models.
Since you are "really particular about the speed," you may find the 15" to be the better choice. However, if your needs are basic, a 13" model likely would work well and it's your decision whether or not the larger display and extra performance is worth the extra money to you.
An objective comparison may be useful:
http://www.everymac.com/systems/apple/macbook_pro/macbook-pro-unibody-faq/differences-between-macbook-pro-13-15-17-inch-mid-2010.html
For performance, specifically, this also may be worthwhile:
http://www.everymac.com/systems/apple/macbook_pro/macbook-pro-unibody-faq/macbook-pro-13-15-17-mid-2010-speed-performance-comparison.html
Hope this helps!