My daughter has dyslexia, too, so I understand–but I also know it's can be used as an umbrella term to describe a host of reading problems, and the exact nature of yours can make a difference in how easily you can remediate the problem. I am no expert, but sicne I have a tiny bit of knoweldge on this, let me use it in answering you--and you can file it in file 13 if it's not useful, of course, lol
If you can read, just not easily, and the "experts" are telling you that in order to create new neural pathways to improve your reading over time practice is the key--i.e. that you need to read as much as possible--then buying a set of manga that you love is one sure fire way to encourage yourself to read in the most enjoyable way possible, right? With the side benefit of improving your skills in reading--great payoff! I understand, and you may have heard yourself, that dyslexic people are often more highly visual than the average (more "right brained" than left I think, but not sure so don't quote me on that) so often manga and graphic novels are a great way to get yourself to want to read.
Plus the manga are more portable, and even if you don't read them you can look at the art anywhere you carry one--in lines, between stuff, at the Dr's office etc. and you don't miss all the little jokes and stuff Oda crams in there that are left out of the anime. You can even go back more easily and recheck something that comes clearer as the story progresses.
If however, it's so painfully slow a process that you can't keep track of the storyline because your decoding takes so long, then stick with the anime. As much as it leaves out, the story is just so good it's hard to butcher it completely, so it's still a rousing good story--and there's a lot to be said for sharing something you love with friends (not that they won't casually pick up your manga if you are with them and leave it sitting on the table in front of them---LOL).
In the end they are both good choices. I've just tried to give you something to put into your lists that you didn't have there already. It's your money and your choice and it's just for fun.
HAve a fun time at the con!