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I prefer the old one - it uses less bandwidth.
classic.checkmytrip.com, however seat maps appear to have been removed from this.
To get a seatmap with the new CMT, after bringing up your booking, at the top right of each trip summary is a symbol. Click this to "open" the trip showing its segments.
At the bottom right of each segment summary is the same symbol. Click this to show segment details.
In there you may see "Seat assignment:" and a seat number. Click the seat number or the symbol to the right of the seat number to bring up a seat map.