The first item on the list for consideration will be the number of players you expect at any point in time. If you've been hosting poker games for fun at your house and attendance has been growing or going up or down, it's a good idea to get a table that can seat the maximum number you expect at any one time.
If you're unsure of the number or want some flexibility, round tables can squeeze in an extra chair or two more than tables with sharp edges. When tables are not fully occupied, round tables excel at "filling" the spaces, since they don't have defined boundaries. Instead of being conscious of the lack of players at your table, you'll be able to fully immerse yourself in gameplay.