Are these both used tractors made somewhere between 2005-2015 or so?
I purchased a DK45SE HST in 2010. At that time, the "SE" was a different platform from the "S" and was the only version that had HST. I believe the "DK45S" was an older platform and very slightly larger - like 1-2" in various dimensions. The SE was newer and initially was HST only. It may have later added gear models.
The current DKxx10 series is different still. Roughly the same size, but a very basic model. The current NX series is probably most comparable to a DK45SE HST but is slightly larger and fancier.
I had zero complaints with my DK45SE HST and sometimes wish I still owned it. It wasn't possible for me to keep it - I was without a farm or acreage for over 2 years - but it was definitely a quality machine.