For the bevelers, it depends on your carving. I was taught that generally if you are going to antique a piece, you want to have the checkering because it will capture the antique and highlight your carving more. Same with the pear shader or thumbprint. However, if you are not antiquing there probably isn't a good reason for it. If you are doing a more realistic floral carving or if you want the tools for figure carving, it makes sense for the beveler to be smooth in most cases, and I actually have a smooth Sheridan style beveler from BK too. I prefer the checkering because most of what I do is antiqued and I like the effect. It probably boils down to simple personal preference.
For the flower center, I personally have the 24 seed centers. Tandy doesn't make anything like them as far as the impression goes. The 12 seed ones do look good, and I'm considering getting one this year for variety. When you look at carvings by the pros where they have multiple flowers, they often have different styles of centers even if the flowers are very similar. Probably more important than the number of seeds is the size of the stamp. On Barry's site, if you click on the pictures of the flower center it will bring up a different picture that shows them in relation to a ruler. It shows you exactly how big each one is. You should pick something that is the right size for the size of the flowers you will most often carve. I think mine are what he calls #2's.
When it comes to the pear shaders, the Tandy ones can be a problem sometimes (IMHO). I have some smooth ones that work very well, but lined ones I have purchased recently have too sharp of an edge so I don't get a smooth transition in the leather at the edges and the tool leaves edge marks. With a lot of practice I could probably make the tool shade an area smoothly, but with the BK thumprint I have it's pretty easy to walk the tool and shade an area. Hence, why I plan of buying a bigger thumprint this year. With Tandy I like to always try the tool first and then buy, but if you don't have a store nearby that's probably not an option for you.
Hope this helps,
Bob