Hilton Canary Wharf
The lounge is far nicer than the DT and for a long 3 week stay I would personally find the Canary Wharf area better for variety of restaurants in the nearby area.
As a Hilton you should get better range or services and staffing and as you mention the wash facilities are closer then this seems to tie in nicely.
Both are quite good but if it was me, HCW.
Enjoy your time in London and do let us know afterwards how you found the choice you made please!