A couple quick points here.....The reality of all of this is that boats are a system. Properly designed as a system where the hull form, rig and appendages are all working together, a fin keel/spade boat will offer significant gains in speed and should offer no liability to motion comfort, seaworthiness or tracking, and may actually offer significant advantages in all cases.
Whether or not a boat has a fin keel has little bearing on whether the boat has a comfortable motion, slams in a seaway, how much it can carry or whether it can tollerate weight in the bow. That is more a product of hull form and keel/rudder shape decisions that full keel or fin keel.
But, and this is a big 'but', most fin keel boats were designed for coastal cruising or some racing and so prioritize performance and cost over seakeeping or motion confort. This means that it takes more time and knowlege to buy a fin keeled boat to go offshore. Since one of the few justifications for buying a full keel is going offshore, a larger percentage of full keeled boats are designed to be offshore cruisers. Often these boats are truely hampered as coastal cruisers so it really makes sense to ask yourself do you really need a full blown offshore cruiser.
When you ask about the speed difference, it is not just a simple matter of speed but also the percentage of time spend sailing. As an example, a few years ago I spoke to a fellow who single-handed a high performance 38 footer from South Africa to the Carribean. He averaged 150 miles a day and used less than 20 gallons over the whole trip (including going through the Duldrums). He left South Africa with a 55 footer which he described as a reasonably modern full keeled boat. As I recall he said that the 55 footer arrived something like 10 days later and used well over 100 gallons of fuel.
I do want to clarify one more point, Boats with a Brewer notch are an example of a long fin keel skeg hung rudder.