Can somebody explain the difference between the different Icebreaker 200 weight merino base layer lines?

Backcountry.com's "chat" service and customer representatives did a really good job of explaining to me what the difference was between Arcteryx's different acronyms (SL, LT, AR, etc.).

I would ask there.

I think the primary differences are the cut of each line.

The Sprint line has a little spandex and (for the top) a large side gusset (you can see this diamond shaped fabric under the arms in the IB photos).

The Everyday line has raglan sleeves where the Oasis seams are offset. Compare the photos of the shoulders for each on IB's site. This is just my (uneducated!) guess, but the price difference probably accounts for (1) the added complexity of the sewing and (2) maybe the ribbed material of the Everyday line lends itself to using a cheaper or imperfect merino. Just a guess, though. For fit, it probably just comes down to which is more comfortable for your wife. Functionally, they are probably the same.

The Tech T's are actually my favorite t-shirt; with a little patience, they can be found on sale. I do have an Oasis L/S shirt that I wear as a winter baselayer; for 3-season backpacking, I'd rather wear a 150-wt baselayer (either S/S or L/S) and a Patagonia Cap4 midlayer.