Thanks to the posters who suggested using cellophane tape to deal with the cap cracking issue on the Platinum Preppys. I'll try that.
As for the Pilot Metropolitan v. Plaisir v. Preppy issue, I think that the Metropolitan feels better in my hand when I'm writing. As has been pointed out, the Plaisir and the Preppy use the same Platinum nib/feed/section assembly. The Platinum section is almost perfectly cylindrical, which makes it less comfortable to me for writing. It also makes it so that the collector cannot be removed from the section for cleaning, although the nib and feed slip out easily. I find the Pilot Metropolitan more comfortable to write with.
I have so far tested two Preppys, and still have more to test out from a batch I bought last year, and so far the Preppy nibs are 50/50. One was very scratchy, the other was all right. The nib on the Plaisir was all right too, and I speculate that Platinum pays more attention to QC on the nibs they put on the Plaisir than to the ones they put on the Preppys.
To me all that the Plaisir does is give you a nicer looking barrel, a basic color instead of a transparent plastic barrel with a lot of verbiage on it, than the Preppy. And it does this at a multiple of three of four times the price of the Preppy .
I'll continue testing out the Preppys that I have, and I'll see if the good to scratchy nib ratio improves, or gets worse. I will say that for the price point the Preppys are so far pretty good, in comparison with other similarly priced fountain pens I've bought in the past.