I have a one-piece the advantage being that in summer I wear my shorts and T shirt underneath and when parked up all I have to do is unzip it and stuff it into my RHS pannier or topbox. It takes about 5 secs to remove.
IN A PROLONGED TORRENTIAL (FRENCH of ITALIAN) DOWNPOUR because of the amount of zips and the fact that the GS position is rather upright with minimal protection (GSA would be better) you do get a little seepage mainly because the air pressure pushes the wet stuff through the flaps and zip. But for normal wet stuff as the others say it is fine.
I ordered mine from the UK (it's your suit size) and had it delivered to my hotel in New York. Aerostich say they do it all the time.
Would I get a 2-piece - No, because it's so convenient to just slide the 1-piece on. It takes less than 10 secs.
You must remember it is an oversuit so you have to wear a sweatshirt or fleece underneath in Spring/Winter. A mate of mine went for the 1-piece having seen mine and has not been disappointed to date.
Your buying decision must be biased by what you do in summer - do you walk around in your leathers in town and at race meets, do you tour a lot in summer, do you really need 100% guaranteed waterproofness, would you like to have lots of air vents to cool you down in summer, are you going to go off-road but don't want to get hot and sticky?
Hope this helps
M