John, I own a 1991 Performer (w/o the gas-assist) and a One-touch Silver that I have modified to the point that it has all of the features (and more) of the One-touch Gold.
I have also cooked on the new One-touch Platinum.
I agree with everything that Don has said, particularly with respect to the additional height of the Performer. How tall is your brother? I am only 5'9", and you might be amazed at the difference that extra inch or two makes, even for me.
The Performer's cart assembly also provides far more stability than the design of the One-touch Gold affords; if your brother will have little ones running around (his own or his guests'), this feature alone would justify the premium of the Performer, or at least the Platinum. Moreover, I live in New England, where we get a fair amount of wet weather, and moisture has never been an issue in my storage bin. I note, however, that my I keep my Performer covered at all times when not in use. Also, my Performer is the older design, with access to the bin via the stainless steel table-top. I cannot say whether the tip-out bin on the current model is as resistant to moisture.
If mobility is an important consideration, I would go with the Gold, and would urge you to throw in the slide-aside and Weber Work-table attachments. That would get your brother a rig that has most of the features of the Platinum (albeit without the lid thermometer and slightly less storage space beneath the kettle), but with better mobility than either the Platinum or the Performer, and better lid clearance from the grill surface with the slide-aside as opposed to the bolted-on tuck-away assembly - for a slightly lower price than that of the Platinum.
The Platinum is a solid piece of equipment. Its cart assembly, while not as substantial as that of the Performer, provides more stability and, as I mentioned above, a slightly larger storage rack below the kettle than the Gold. It also has the two (smallish) side-tables, the bolted-on tuck-away lid assembly, and lid thermometer, all of which the Gold lacks.
However, I understand that the Platinum also comes with a price tag of about $250. Once you are up in this range, the extra $80 or so your family would have to pony up for the higher cooking surface, the more useful single large table-top work surface, the greater stability (with locking casters), the storage bin, an even larger storage rack below, and the gas-assist feature make the Performer a better value than the Platinum.
If you expect that your brother will frequently have occasion to cook for larger crowds, and would only have the one charcoal grill, I suggest that the 26" is a better option than any of the 22" models, including the Performer.
Whichever option you choose, your brother will be receiving a righteous gift.