I think you may find the majority leaning toward the Lyman offerings due to quality, etc.
That is not to say however, that Traditions are no good, in fact I like both. A lot depends on whether you customize them any or not, or whether accuracy is a primary concern.
You can do a heck of a lot with a 120buck Traditions kit-even making a Pioneer into a semi-fine dualer.
Or get one already built, and leave it as is.
For the quality, I'd have to say Lyman.
There is one downfall with there pistols though:
They only have one model. Only difference is caliber.
The .54 does have an edge over the Traditions offerings if you like a slightly bigger bore.
For some fun and enjoyment with building or just plinking around-go with Traditions.
For accuracy and "fair" Italian quality-go with Lyman
Both brands seem to make different people happy in different ways.
Since you have a Plains rifle-you may wish to mate it with the companion pistol. This could be a big decision maker in itself-and one that I would really consider.
If your rifle is a .54-get the same for pistol-cast same balls.
Hope this helps.
-sleepy-