Hi Andrew,
First of all let's look at widths - Geckos are wider, thinner, fairly narrow tails - so the 120 you tried was probably the older shape with 76cm or the newer 122 with a width of 77cm. That is indeed fairly wide, vs classic older 120L shapes. The Blasts on the other hand have narrower centres, wider tails and the 115 has a width of 66cm, the 130 only 70cm - so both considerably narrower than the 120 Gecko you tried.
So if we are looking at a allround shape to replace both the 103 and your Mistral, then I think the Blast 115 could be a very good option. If you think the 103 you have is still good enough and a more light wind shape needed, then go for the Blast 130 - yes 2020 models are all also with equipped with foil approved tuttle boxes. To be honest, if you are still working on exiting the gybes with speed/floating back, then perhaps take the Blast 130, it will still feel fairly "small" at 70cm width and will help you to learn the gybe technique. Normally I do reccomend anyone looking at improving their gybing to go for the Gecko range, as these are specifically designed as gybe learners. You could also look at going for a Gecko 110 in that case, at 72cm width. But this does not have a foil box. Generally though I would avoid trying to learn foiling with boards under 120-130L, as the narrower, smaller boards are quite hard to change down to.
if you share the details on your 103L board, then it might help us also to determine if you should take the leap straight to a 115 Blast or 130? Width/style of board/your sail quiver?