Most ribbers hate anti-foul and rightly so. It's not as if it works anymore anyway. It's like antelope tinkle these days and will slow you down, but i don't notice the price dropping. Once you put it on, your hull is marked for ever and will look naff.
Be careful with very high pressure washers. They can (and do) damage tubes and seams. It does pretty much remove the weed. If it's more tenacious, use "Wipeout" as well (Oxalyic Acid). It works very well.
20 knots knocked off speed? Only if weed longer than Neptunes beard
Even a thin slime film will knock off up to 5-8 knots and increase fuel consumption accordingly.
How much fouling depends where you moor. In a strong stream is best. In a closed marina is worst. In a strong stream with some fresh water element is even better.
Other advantages of dry berthing or trailer sailing include easier maintenance, ability to flush through after each use, overall cover, cheaper.
Pressure washing does NOT remove all the barnacles, again, depends where you moor. The little tiny buggers are the worst.
Against trailering is you will not be able to keep a set of brakes on your trailer for very long unless you use a fresh water flushing system (available now from some trailer specialists). Washing down with a hose some hours later will not work. Also, (and this applies to boaters who use Itchenor Hard at high tidem half tide etc., if you immerse your tow vehicle more than just the bottom of the wheels, it will disintegrate after 6 months. The best time to launch at Itchenor is low tide when the shingle shelves steeply and you don't have to immerse your vehicle. The only reason I say that is because I watched a brand new Nissan Navarra up to it's door sills in the oggin launching a boat. When I asked if this was wise he said "No problem mate they're all corrosion proof" As I write this, the little rusticles are eating his pride and joy from the inside.
Personally? I hate the thought of these fat cat marina owning bastards thinking that everyone who has a boat is stinking rich getting ANY of my money. They even want £150 pm to keep a boat on a trailer! Sod 'em!
Sorry to be lengthy, but these are all considerations and I hope these real life experiences help in some way.