I have skipped this debate by using small potted plants in the place of a tree. Some years I use a small potted evergreen but this year I went in a different direction. This year I started planning ahead for what I want to plant in the spring. One of these plans is for more lavender in the birdbath garden. From there I went and looked till I found a small potted lavender plant already trimmed into a tree shape.
If you do not like lavender a rosemary plant works very well. Other plants such as small evergreens work great too. There is really no rule here on what type of plant someone may want to dress up. It could even be cactus if that is what someone wants to use. Personally, my only rule is that I plan to relocate the plant outside in the spring.
The secret to turning a potted topiary into a Christmas tree is just decorations. Each year I reuse the same miniature ornament set I picked up at a craft store years ago. This set came with tiny plastic ornaments, battery powered LED lights, a plastic bead strand, and a little star for the top.
The plant sits in a bowl to catch any water that runs through. It gets moved occasionally to sit with the other plants in the grow lamp. The rest of the time it sits in the dining room being festive. It also has its own hardware cloth cage to keep the cats from digging in the dirt.
I have noticed it has grown a bit since I first bought it. There are shoots of new growth sticking out at the sides. I could trim these to keep the plant in a tree shape or let it go since we are only a few days away from Christmas. After the holidays I will remove the decorations and store them for next year. Then I will put the plant in with the citrus trees under the grow lamp. As soon as it is warm enough, I will plant it in the garden.