Hello Joe,
I think your question has already been quite well answered, so just my opinion
If you have some older Loks you wish to upgrade with new decoders, ESU is the way to go for ease of programming etc but you should also have an ESU Lok programmer ! That said, the Marklin decoders are cheaper and there is the decoder tool available but they can also be programmed with the CS2, albeit rather labourously compared to the ESU Lok programmer.
I am lucky in that I have good friend Dennis the "Train Doctor" to do all that for me !The Ecos makes fine tuning them a breeze, no worrying about CV numbers etc I have a CS1-R, an Ecos in a Marklin Case, as they were made by ESU but delivered to Marklin, and us poor suckers, the Marklin consumer, in a somewhat handicapped state, that is until the "Reload" was made available by ESU.
Hence Marklin's brave, but logical, move at the hand of the investment Bankers, to commission the CS2
Like Tom (HO) I enjoy using my CS1-R and I am very comfortable using it, but as yet am only "playing" as my layout isn't built yet and if it weren't for MFX+ ( a very cunning idea by M) I would not consider the CS2 because of it's sensitivities, not to mention higher cost.
I have a number of Loks with MFX+ and as my layout is not going to be large, except for the BTW which will dominate the front of it

I considered MFX+ something to add a bit of operational interest (read play value

) to my layout and it will be built and wired accordingly
One thing if you decide for the CS2, do not under any circumstances connect it directly to the tracks of your layout !

Always run track power thru a Booster, and for MFX's sake a Marklin 60174 booster

I believe this is absolutely critical if you have any M track on your layout
Reason; if you get a short circuit on the layout tracks, it will cut out at the Booster, and not fry the Software in your CS2

This Booster scenario, is however, also recommended for the Ecos as well, by those that know
I will have three Boosters for my 2.7 x 2m "L" shaped K Track layout. One for the main part, one for the BTW, (turntable has 16 spoke tracks and a 12 Bay M72883 Ringlok Shed, as well as the Marklin Digital Coaling Station. The third booster will power the "Schatten Bahnhoff"
All that said, your digital decoders for Signals and points etc are ok to be connected directly to the CS2/Ecos using it's built in booster, so everything is on a separate circuit and if a short occurs on the layout you can still operate points and signals, and the trains on other boosted circuits !
So that is my $NZ2.00 worth (approx. $US1.50, Euro 1.37 or GBP 1.01)