Of the 3 I am aware of:
KB boat park is great with it's location near the entrance to portsmouth, pretty much next to the wightlink terminal and easy access in to the solent. But you do have to drive right through the middle of portsmouth to get there.
Trafalgar is at the top end of portsmouth - near port solent. Bit further to go, but portsmouth does have a decent speed limit so while you're not on the plane, it doesn't take forever to get there, and is an interesting run through the harbour.
Southampton dry stack on to the itchen is decent, including an excellent on site school (tec rec coaching). Downside is it's a bit of a drag up the itchen (6 knots), but on the upside, if you fancy a bit of a go and the solent's a bit choppy, you're launching into southampton water, which in many cases is enough for a bit of messing around on the water, esp if you have kids - ie you can give them a run from the itchen down the hamble for lunch without getting involved in the solent snot.
And as a curve ball - I launch both my boats from drivers dry berthing and keep one of them there.
They're not a forklift setup, they are a crane & boat mover - so you pay storage and each launch is extra.
But as an infrequent boater, this works for me!
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