Männlichen-Kleine Scheidegg OR First-Bachalpsee?

Both are gorgeous but my first pick would be Männlichen-Kleine Scheidegg. This route affords spectacular views of the Eiger, Mönch, and Jungfrau peaks and terminates in KS -- the base camp for Eiger ascents. The views here across to the glaciers are fabulous, as is the food! The First-Bachalpsee route is beautiful as well with a view of the lovely small lake and, on the return trip, a very photogenic view of the Wetterhorn peak. The ride up/back to First on the gondolas is also great fun.

You can combine the Männlichen - KS walk with a visit to Wengen as you end up starting, returning there. Likewise the First-Bachalpsee trip can be combined with a visit to Grindelwald. IMO, Grindelwald is nothing special -- touristy and full of traffic and T-shirt shops so I'd skip it or see it as you walk through town to get to/from the First lift.

Be aware that neither trails may be open if there is late snow. Also, this year the Wengen to Männlichen cable car is not scheduled to open until June 10 due to maintenance. These things are usually done on time but you should check to make sure it is running before you go (the Lauterbrunnen or Wengen tourist office will know).