Experience is the best advisor. Just in case anyone else is interested in this issue, here it goes. Trek's '04 and particularly '05 Hybrid series unfortunately suck in stays tyre clearance. The "Trek 7300 FX S" (Small size --why should it change with frame size?) hardly has room for a 34-622 (labeled 700x35c), let alone plus fenders. The room between the seatstay bridge and the tyre is a mere 7-8 mm! Down behind the bottom bracket, distance to the bridge (not a tube, but a plate) goes to nearly 1 cm and hardly any allowance for side increase in size. As for the "Scott Sportster", I have installed a gauge-measured Panaracer Smoke 45-622 (700x45c) with full mudguard with no problem. Provided that the wheel is perfectly true (usually the case), it could admit up to 47-622 with 4 mm clearance on either side of the tyre. Needless to say, the Scott is the one I bought (thanks to Carlos, my nice dealer in León, who kindly brought both bikes for me just to compare). It is a pity that manufacturers tend to go different ways from users (at least from my experience in Spain): tight tyre clearance instead of wide room, aluminium instead of Cro-Mo steel, suspension instead of rigid forks, and so on.

Cheers to all.