Technically it depends on how you are going to play it. If you are going thru USB, DSD128 is the highest sample rate supported by current DS firmware. If you are using the PS Audio Bridge input I’d stick to DSD64 (tho DSD128 will work for many.) If you are using S/PDIF or AES3 DSD64 is the max. On some systems you can do DSD64 over TOSLink (but not all sources or TOSLink cables will support it.)
Oops I just saw that you have the DMP - tho I don’t have experience with the current DMP software, with the software I have I can only do DSD64 well. Certainly if you try DSD128 and it works fine for you, then that’s fine.
At some point you’ll likely be able to send DSD256 to the DS via I2S if your I2S source supports it.
Some sites give you all of the lower resolutions free if you get a higher resolution version. If so, depending on your disk storage available, your willingness to deal with multiple resolutions of the same material and your budget you can make a choice that suits you.