If you only use Roll20 for the map, this is the map
Many tables use a full-featured VTT for one thing: a battle map with tokens. Tabletop is that one thing, done in 3D with real miniatures and light, and it leaves character sheets, dice and macros to the books and the people at the table.

Side by side
| Topic | Tabletop | Roll20 |
|---|---|---|
| Maps | 3D scenes from a built-in realistic library; build in minutes | Image maps; a marketplace of art |
| Character sheets and dice | None, by design | Extensive, per system |
| Combat tracking | Initiative, HP, conditions, movement per turn | Turn tracker and sheet-driven automation |
| Measuring | Feet with diagonal rule, waypoints and height | Ruler on a flat grid |
| Remote play | Not yet | Yes, with voice and video |
| Learning curve | Five minutes | Considerable for the DM |
Roll20 is a trademark of its owners and is not affiliated with Tabletop. Feature notes describe the other product in general terms as we understand it at the time of writing; check its site for current details.
Less to prepare
No sheets to build, no macros to fix, no marketplace to browse. Drag a room, dress it, drop the party in. The starter scenes exist so that session one is a click.

A map players look at
Torchlight in a corridor, a hearth with a fire in it, a moon over a graveyard. Realistic materials and animated minis are the whole product, not an add-on.

Where Roll20 is still the better pick
Fully remote groups, tables that want automation, and anyone with a library of purchased content. Tabletop is for the map and the fight on it.
Questions
Can I use Tabletop alongside Roll20?
Yes. Many groups keep sheets and dice in one tool and put the battle map on a shared screen in another. Tabletop on a TV or a screen share works well for that.
Does it have a compendium?
No. It is not a rules product.
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Ready to put a map on the table?
Sign up in a minute. Five starter scenes are waiting on the welcome screen.