It is a very difficult question! I hear that the Mehano 66 is an excellent model, but of course a Gützold is a Gützold...I chose a 219 because you offered it twice which to me means you would prefer to end up with one of them! Also has Mehano yet released your most preferred livery on the 66?
Obviously this is irrelevant if you collect some models for display as well as the layout, but I always try to focus on purchasing complete trains i.e when I saw the Hobbytrain Taurus model in new ÖBB livery, I thought also that Roco was about to release the Eurofimas in new livery so I can make a complete train if I buy with both. Were I to buy lovely steam locomotive like a Semmering 52, even though the recent Minitrix model is very nice and state of the art it would look out of place with all my epoch IV & V models, and I would have to source some suitable coaches. (Located in a hard to find Minitrix set from a few years ago!)
This approach means that I balance the locomotive and rollingstock amounts, (although one can never get enough freight wagons to be prototypical without a sports hall to build the layout in!!) and though of course they are never fixed combinations it is sensible to balance them in order to get maximum pleasure of variety on my layout. Nice though fifteen locomotives look parked in the depot, I feel that it is better if they are used as much as possible like on the real railways, by having a train ready for most of them. Also I imagine that If I want to run the layout on DCC with automated trains I know that I can have a wide selection of prototypical trains available at the same time, without coupling and uncoupling and swapping over. Perhaps the trick here is to have huge storage sidings so that no locomotives remain in their boxes?
I confess to be a bit sad - I have a database with all my models listed in, locomotive and wagons, for each train, however the reason is only so that I know where I need to focus my 'eBay eye' or scour the shops for a few more Shimms or more 2nd class Eurofimas - why does one always have too many 1st and 1st/2nd and restauruant coaches to be prototypical?? Always on ebay one must buy a 1st and 2nd together in an auction - but I don't want the 1st - it suits the collector who wants one of each but...grrr...I am not such a collector!
If one has 20 locomotives and 200 wagons/coaches say then without some kind of record it can be confusing what to buy to strengthen the collection, either for display or for operation.
However the pure collector does not have these constraints on efficient usage, and always there is room for one more top class model on his mantlepiece, so each must decide what is a priority.
I hope that you are able to decide ME 26-06, and are not forced to buy one of each!
Goedel