First, I imagine you'll be pleased with either of these two properties so there is likely no wrong choice here given you'll be close to the tube and in nice rooms in some of the better London Hilton chain hotels...

I haven't stayed in the suite/apartment so can't comment on those specific rooms, however, i have stayed at both hotels and can add the following which may be of use;

Tower of London hotel:

-less hectic than West end for breakfast but still busy.
-very modern, white and pale wood design so feels clean and bigger, more modern than the West end
-sky lounge bar is on the top floor and has nice views
-close to some tourist sites and quiet

You may wish to ask the hotel if the suites here have separating doors in the suite room? Many other former Mint DT's I have stayed at in the UK do NOT have separating doors between the living room area and the bedroom...if this is important to you. West end will have proper doors separating the rooms. (for a one bed suite picture review see my recent posts in the DT Manchester Piccadilly thread to see the archway separating the rooms).

West End hotel:

-better location if you want to spend most of your time around covent garden area or walk to the central London areas instead of using the tube
-breakfast can be very, very busy but is good
-rooms are more classic in design rather than modern as some of the newer DT hotels. The hotel itself (lobby, bar, etc...) is quite nicely done up though, however, the rooms are not generally as nice as the Tower of London.
-bar is on the ground floor off to the side of the reception and lobby so less dynamic
-lots of smaller shops and coffee shops/restaurants nearby so won't need to go far.

Again, splitting hairs really, both are nice but have different 'feel' to them. If you want modern, minimalist look and a bit of bling then the Tower of London is nicer IMO...if you want location and are looking to walk more than use the tube...I would choose West End.

Hope these points help.

Enjoy your time in London!