Neither are considered a quality product nor a well thought of vendor, most pro's will tell you many issues with either if they even attempted to use them.

Thanks, @millworkman . The reason I am looking into sash replacements is that I don't want avoid the cost and hassle of full replacement, and really don't want to lose glass space with a pocket/insert. But perhaps I am wrong with my assumption that any pocket/insert will lead to noticeable glass loss. Are there pocket units with an ultra-thin frame so that glass loss is minimized? Maybe something from Marvin?

What does the current window consist of right now. Pictures? Inside and out.

No storm windows. Typically you would see a single pane wood with a storm which makes the replacement window actually FEEL larger in total glass because of the lack of horizontal encroachment on the viewing area.

If you measure from the edge of stop to the edge of glass, that will tell you a base measurement. As a point of reference, thin vinyl replacements will have about an edge of frame (i.e. what is buried behind the stop) to edge of glass measurement of about 2.75".

I think in this case, you measurement is probably close to 2-2.25". Meaning you would loose about 0.5" on either side with a replacement. Probably not as much as you were thinking and to be perfectly honest, given the suspect track record for sash kits and their general lack of air-tightness, I would go with the insert replacement (i.e. pocket set).

Thanks, @Windows on Washington Ltd . I checked the measure you indicated, and it’s 1.75.

I prefer to have wood (or wood-look) inside. In the event that i stay away from the sash packs and go for an insert, what would be the best insert with the narrowest frame? maybe Marvin Infinity with an Everwood inside?

Meh. I'd rather the elevate with actual wood.

Thanks, @Windows on Washington Ltd. would you say that an Elevate insert leads to substantially less glass loss than a wood/clad insert like Marvin Ultimate or the like?

Thanks, all. An additional question: does anyone have experience with Sierra Pacific sash replacement kits? How about Windsor kits?

Best one of your group is the Marvin. Manage your expectations. Sash kits are going to get much, on the average, more negative feedback with regards to smooth operation and draftiness.

Thanks, @Windows on Washington Ltd. i just learned Marvin has discontinued its Tilt Pac..

"does anyone have experience with Sierra Pacific sash replacement kits? How about Windsor kits?"

Poor fit and finish, poor quality control, poor customer service for both.