The Super T is a much plusher fork, but it will also plant a lot of weight on your front end and it's a triple clamp, so this translates very likely into stress that a Mantra frame can't be prepared to handle. Even if it's a "Trek era" bike and in some people's minds it should be burned on a stake (it may hurt the memory of Dolomite and other excessively painted bikes), you wouldn't wanna break what is basically a nice frame.
The Manitou isn't much of a fork by today's terms, but I reckon it could work on pavement and for lightish off-road use.
I think a modern fork should do the job without adding weight penalty and without harming the frame. How about a Reba Dual Air in its 120 mm version, or an older 32 TALAS (even though it is overcomplicated on the inside), even a Recon with U-Turn and aluminium stanchions? A Minute?
1990 GT Karakoram 20" Purple Haze (urban project); 1993 GT Karakoram 20" Ferrari Red (classy beater); 1993 GT Karakoram 14.5" Black Light (almost original); 1994 Breezer Jet Stream 18" (unlikely story); 1995 Winora full suspension (a fine riding mistery); 1997 Kona A'Ha (anniversary edition); 1997 GT Arrowhead 18" (poisoned gift); 1998 Stevens 6.1.2 (true warrior); 1998 GT Moto cruiser (just google it!); 1999 Santa Cruz Chameleon (the heartbreaker revived); 1999 GT Lobo 1000 (retired with works Super T); 2000 GT Lobo (turns original); 2001 Klein Attitude Comp 19" (ultralight project); 2001 Cannondale Jekyll 1000 SL (better that a lot of "fresh" bikes); 2003 Intense Tazer (close to factory team spec); 2003 Intense M1 (old dream on hold); 2007 Crupi Pro XXL (pumptrack teacher)