Engine, what?
Both Lycoming and TCM have a very poor record in honouring guarantees on their engines. The main advantage of going for a factory engine is that you can continue flying while you wait for the new engine to be delivered. Norvic have a very good reputation but are sometimes very slow - can be in excess of three months or more. I'm sure many will comment good and bad on this point.
We are fortunate in the UK, perhaps owing to our Formula 1 history, that we have a wealth of expertise here. I have had four engines rebuilt in this country and they have all been excellent jobs at a cheaper cost than a 'factory overhauled' engine. By the way, try getting either Lycoming or TCM to explain the difference between their 'overhauled and 'rebuilt' engines, other than to quote a large price difference.
Another plus for having your own rebuild is that you are consulted all the way as to what is being replaced and why and with what. The choice of Superior cylinders for instance. Millair gave a very good service but i don't think they are still in business. They provided a service such that for £500 they would strip the engine and give you a fixed quote and date for rebuild. If you didn't wish to accept their qoute they would for the money put it back together and return it to you. Both Lycoming and TCM will not by the way accept your old engine as a core if it has been split by anyone else - be warned, you should check.
Brian Mills, near Cambridge, charges a fixed fee for the labour and you pay for parts yourself, at the cost.