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tube trellis. and its not necessarily "less flexible". all frames flex some. its just flexible in different ways. the diamond neck tube frame on the 250 is an engine supported bottom and has what is essentially an A arm from the rear subframe up to the neck. so it doesn't flex a whole lot forward and back or up and down but flexes a lot side to side. good attributes for a street bike where you're primarily hitting things straight up and down like speed bumps or potholes. perimeter frames, whether they are made with boxed aluminum (common for todays sport bikes due to weight) or steel tube trellis, are designed to have better support laterally. most newer perimeter frames use the engine as the bottom support as well as an internal lateral brace. but some older frames (most of the perimeter frames made with boxed steel sheet, which is much softer and easier to deform) use down-tubes to support the bottom of the neck instead of the relatively brittle engine casings of the time. many of the downtubes on steel box frames were also bolted to the sides of the bottom of the engine for extra lateral support. anyway, because of the design of the perimeter frame, it allows more vertical flex of the neck, but much less lateral flex. you'll notice the forward mount on the gas tank for perimeter frames is always made to handle at least an inch or so of frame flex.
tl-dr: there are tons of variables, and what kind of flex you want in a frame changes dramatically depending on use.
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