Hi Henrik, hmm, it's not only about the outline, but also the bottom curve and shape. So the Gecko has a flatter overall rocker than the Blast, plus a wider centre and relatively tucked in tail. The Blast has a higher rocker, shorter flat section in tail, more narrower parallel outline. What that does, is that the Gecko sits flatter on the water, in the same volume size it will plane earlier, stay flatter and lower on the water, which is easier for it's target user. The Blast will ride higher out of the water, turn tighter, feel more lively. But will be a bit more technical to sail if you are an intermediate style rider.
An experienced rider, who is the targeted customer for a Blast, will have more fun in chop on the Blast. But a less experienced rider might find it is too difficult to ride if it's too lively.
The different widths question in chop between the boards in a Gecko between 110 and 120, means you would probably find it easier on a narrower shape, so long as your skill level matches that volume and width size. Just be careful between classic volumes vs Blast and Gecko - the Blast´s are actually pretty narrow vs a Gecko, in 130L the Blast is 70cm wide, in the Gecko 135 - 79cm?
Might need a bit more info on your skill set to help advise further? Weight and sail sizes/local conditions and current board also helpful?