A spell (beyond your Priority allocation) during character generation costs 5 karma; a Power Point costs 5 Karma. I would likely allow (or request) that a single spell slot be allowed to substitute for a Power Point.
The problem with this system comes into effect after character creation. A player can easily replace that lost spell slot for 5 karma after character generation. They cannot buy a power point after character generation. This means that someone could easily sacrifice 6 spell slots for 6 power points, spending 0 karma at character creation, and then spending 30 karma later to replace those spell slots. It is, in effect, allowing a player to buy power points after character creation.
Thus, the argument by many is that if you allow spells to be traded for power points, why not just allow power points to be bought after character creation? My argument is that mystic adepts could never have 6 power points and 6 Magic points, so why should people complain if it costs a bit more to actually be able to achieve that desired goal? As I type this, my two week old daughter is doing the same thing. She cries when she's on her back because she wants to be on her side, but then she cries on her side because she's not on her back.
One (admittedly relatively minor) counterargument to be made is that, with the rules as they are now, it's very expensive to make a Mystic Adept that's high on the Adept and low on the Mystic side. You have to buy the 6 power points in that case, but can't
not buy (some of) the spellcasting power that entails, so you end up paying out of your nose for something you don't want.
The rules as they stand are excellent for a quasi-magician that wants a bunch of powers on top of that package. 5 karma per PP is certainly not overpriced in that case. But something to help the Adept who wants some minor spellcasting on the side wouldn't be overpowered in the least in my opinion. Problem, though, is anything that helps the latter will probably overpower the former as well. That's why I like the 2 spells per PP option, btw; no sane "magician-adept" would take it, but less caster-focused characters might.
Mind you, the argument that MA's in former editions couldn't have both at the same time doesn't really work for me. While SR5 should certainly look back to earlier editions to inform flavor and perhaps guide the rules, those rules stand on their own and should be balanced on their own. As others noticed, MA's in former editions were kind of underpowered; them working differently in SR5 shouldn't be an automatic strike against changing them further away from that. MA being fine (and possibly too powerful) in SR5
is an argument against that, but that should stand on it's own merit, without consideration for how SR4 handled it.