Hello Mary-Kay,
We also use SketchUp; it's user-friendly and provides great tools for 3D customization. I like that you can share and download 3D models from the communal model library. It's helpful when you are dropping in furniture in your layout or need a shape that's tricky to build. I would give the free version a trial, it has enough tools to get you started and familiar with the app.
Another program I like to use for sketches is Photoshop. I like to collage on top of a photo to get a feel of the things in space. This program needs time and dedication to learn tho. Im attaching a sketch I did with Photoshop so you get the idea.
I took a quick look at Artplace, it seems to be an app to mock up 2D works. Not sure what your needs are, I suggest you get free trials to see what's your best match for your project. Good luck!
------------------------------
Grisel Marquez-Razon
Registration and Exhibitions Assistant
Mingei International Museum
San Diego CA
------------------------------