Generally, the Tricaster products have all the bells and whistles, including the kitchen sink and all the plumbing. But VERY software and computer system dependent and not inherently as reliable as a one-piece, all-hardware solution like ATEM.
Features: Tricaster wins (if you need all that)
Price: BMD ATEM wins (unless you must buy extra hardware to provide other features)
Reliability: BMD ATEM wins (although the newer Tricaster stuff looks like they are getting better than glorified PC boxes).
Size: BMD ATEM wins (again, unless you need more features that take space, like recorders, etc. etc.)
I would much rather rely on an ATEM on the road than any Tricaster (which are essentially very fancy PCs). Although the more recent Tricaster models seem to be more industrial-strength.
IMHO, it depends A LOT on exactly what your requirements are and how they fit what is available from Newtek or BMD. Since you provided no details, we couldn't even guess how that decision would fall.
I'll agree with what Richard wrote -- we have both an HD Tricaster and several ATEM units -- but would caution that Blackmagic is not particularly good with customer support, and that they tend to sell functionality that doesn't yet exist. For example, the ATEM Television Studio was sold for the better part of a year before they released a firmware revision that let it actually operate in standard definition, even though SD was in the sales literature from the start. I'm still waiting for a firmware update to correct captioning problems with up-down-cross converter cards I bought last December.
My point is not to rant, but rather to suggest that you make certain that whichever product you select actually performs all of the functions you need and expect before you write the check.
For what it's worth, you might also want to take a look at the Ross Crossover or Carbonite: they are very stable, less clunky than the Tricaster, and perform automatic format conversion on the inputs. Ross technical support is about the best I have seen in 30+ years.
Engineering Project Manager
Newport Television, Northeast