You can't go wrong
I think location location location. Where do you live? Check shipping cost and there may be tax of duty issues from Canada, which they can address. Call both of them and talk to them.
I have called both companies and liked what I heard, and from what others say I have heard almost 100% good things about both. You can't go wrong from a quality standpoint. I would lean towards Matt if you are on the East side of the Mississippi river.
The thing that impressed me about Mattituck, besides price, is they are real flexible. They will delete, add and mix-n-match options or ECI/Superior parts to best advantage. Superior parts are a little more expensive in general than ECI but not necessarily better. Mattituck recommends/prefers the ECI cylinders for example, because they have better resistance to corrosion. Nothing wrong with Superior, but they know personal planes fly so little that corrosion resistance is a good characteristic to have in the Jugs. Unless you are flying almost every day or other day corrosion is a factor, espically in some wet parts of the country.
This is where buying direct from Superior falls short, price and flexibility. If you buy a straight up engine, you are fine, but with extras or options they just add on to the price and give very little or no credit for the parts being substituted, like electronic ignition. Mattituck has a good site with credits and prices of a wide range of options.
You will pay a premium to by direct from Superior, but some people find their build-it-yourself class a real learning experience, and after all that is one of the main reasons for building a plane, learning. However it will cost more to do the class.
I would check around. If you are in California (Eagle Engines), Oklahoma (America's aircraft engines). Shop around, there are many good engine shops. I also want a facility that has an engine dyno. Any good engine shop can build up a kit engine from new parts for you and do a nice job. As far as doing-it-your-self, I checked the price difference and the amount you save is small, especially when you factor in warrantee and the benefit of an initial Dyno break-in, both of which you don?t get with a do-it-yourself build. I have a A&P/AI friend and ended up buying a used Lyc O-360A1A. The core was thankfully good and got it cheap a number of years ago. My buddy and I did the overhaul thru ECI. I have about $11,000 all up into it, new everything except cylinders which I overhauled with new valves, pistons and rings. However I would like to have her on a Dyno before flying, which we could not do. There is an aviation college about 100 miles away with a dyno, and I am thinking about bringing it there, if I can talk them into it.
Good luck, get your checkbook out, George