What are you looking to do. They're all interesting places. Both Neuchâtel and Solothurn have interesting looking castles in town.

Neuchâtel has a pretty nice lakeshore, and the Jura mountains nearby, including Creux de Van. You can also walk up the L'areuse Gorge. And when you get to Noirague, there is a shop in the old train station where you can sample some Absinthe. The chocolates there are pretty good.

Solothurn has the Einsiedelei nearby, and you can travel up the nearby Weissenstein mountain.

It's a pretty nice part of the country.