2" or 1.25" diagonal for MK67?

i've heard a 1.25" diagonal won't bring most eps to focus. true? if so, am i better off with a 2" diagonal even though I only have 1.25" eps (pentax xls) maybe I would add a 2" EP later for wide field use. but for now. am i better off (weight wise or for any other reason) with a 1.25" diagonal and some kind of extender?

MK67s of recent vintage (last ~four years) are indeed designed for 2-inch diagonal use as a standard (i.e. focal plane sits around ~four inches behind rear of focuser housing). A two-inch extender (both diameter and length) will set the scope up for use with 1.25-inch diagonals. I use the Intes 2-inch extender with a Lumicon 1.25-inch diagonal, and this set up -- so far, anyway -- has accomodated every .965- and/or 1.25-inch eyepiece I've tried. That list includes more than 30 eyepieces of various designs, ranging from 4mm thru 35mm.

With max-field 2-inchers, an MK67 may gain an additional ~half degree (50% wider) over a max-field 1.25-incher's field.

I wouldn't say you'd be "better off" with *either* configuration, so long as the diagonal in question is excellent.

Since you're thinking "maybe" about a 2-inch eyepiece later, perhaps it makes more sense to "maybe" add a 2-inch diagonal at that time -- in the future -- rather than right off the bat.

Best wishes.
-Dan