The Vans offering is an authentic Lycoming. The Mattituck offering is a Lycoming clone, probably making extensive use of ECI components.
A new engine is a very significant (and expensive) purchase decision so you should take the opportunity to do some research via the VansAirforce archives. The following thread might be a good place to start.
http://www.vansairforce.com/communit...ht=ECI+problem
Personally I would not be worrying too much about saving a couple of thousand dollars on a purchase of this importance....I'd go for the best because your life will depend on your engine. Shopping for an engine based primarily on price is like trying to save a few dollars by choosing a cheap heart pacemaker.
Mattituck is a good company. Why don't you ask them to give you a price on an engine built from Lycoming parts for comparison.
I recommend you also check the prices from AeroSport Power. They are an excellent company as well. You can go here to get comparative prices on engines with ECI parts, Superior parts, and Lycoming parts.
http://www.aerosportpower.com/prices.htm#NewEngines