Rhythm is the default, and generally more popular, game mode, but it's up to you whether your map is optimized for Rhythm or Force play (this will depend on which mode you playtest more). Either way, you won't actually need to put much effort into making the map work in both modes, as most motions will wind up being comfortable in both.
Here are a few guidelines you can follow to make sure your map is fun for both play styles, even if you do minimal (if any) playtesting of the other mode.
When making a Rhythm map, keep the following in mind so that Force players can enjoy the map, too.
Force scoring is based on velocity through the note, so sections that do not encourage movement create awkward gameplay for Force players. Try to keep enough motion throughout the map so that Force players don't have to resort to awkward strategies.
It can be difficult to keep your momentum and balance when hitting two handed special patterns in Force. Make sure that two handed specials are a bit more restrained than other notes so as not to mess up a Force player's balance. This applies to Rhythm as well, but the issue is more pronounced in Force.
This applies to any two handed pattern where both hands are close together.
Keep the following in mind when making a Force map so that it plays well in Rhythm, too.
Sections with sparse note timings (such as a note every two downbeats) might play well in Force, but not have enough motion to be fun in Rhythm. Players who optimize their Rhythm score will move as slowly and early as the game allows them to, which reduces the impact of notes.