If the Dixie SMR is the Miroku-made one, Dixie no longer has parts but it is more authentic than the T/C (parts easy to find). Around here (rural Missouri), those prices are a bit high, but finding anything sitting in a store is rare. Being able to physically examine a used gun is really good.
I've had Dixies and T/C's. Enjoyed them all. Bore condition is, to me, more important than the rest of any individual M/L rifle if I intend to shoot it. Replacement barrels are easy for the T/C. Replacing a bad barrel on the Dixies would be difficult.
I would offer around 75 percent of the asking price for either the Dixie Mountain Rifle or the TC.
Which one would depend on what I intend to use it for. If plans are for shooting in competition requiring patched round ball as much or more that hunting then the Dixie Mountain Rifle would be best ( my opinion).
Dixie parts for that rifle are rare BUT L&R locks and parts for them are easy to find.
Converting the " DMR " from percussion to flintlock and back is no more effort than dropping a $200 L&R flintlock that the mortise has already been (presumably) fitted for, since it has the percussion L&R mounted in it, after unscrewing the drum, then screwing in the flash hole liner that is threaded to fit right in. This was a feature that helped make these rifles popular in their day. The barrel ( if it was cared for in the past) will shoot as well as any other production rifle of that time.
I've owned the 50 cal DMR in the past and still own the Squirrel Rifle version with an L&R flintlock on it. When last listed by DGW, they were over $900 per copy with shipping/ insurance being around $100 even then.
On the other hand, if you're looking for a hunting rifle where a conical bullet will be your go to answer then the TC is a better choice. Owned and loved a .45 TC Hawken for several years using it for that role.
Hard to beat how that gun handled from a deer stand and in thick woods. However, it will never be a target gun without buying a $400 or more 1 in 60 twist barrel and even that won't give you that sight radius that the DMR has. But your tool to fit the job.
Just my 2 cents