I'm going to argue against this because, Risky Mission B is as much of a milestone as Risky Mission A is. Someone applying it as milestone XP should be giving it on both situations, not just situation A. How is that a hard-coded milestone? DM just goes "Yeah, you've completed the appropriate challenge, ding"
I'm referring to situatuions where the DM has a) decided and b) told the players that the milestone is Risky Mission A, period.
I agree with you that - if one must use this system - either A or B should be good enough.
As compared to individual XP which is... Literately just MMO XP where the incentive instead becomes "How can I easily get XP" and trying the oft-mentioned silly tactics of "Oh I just mass-killed some things through an inventive tactic, how much XP do I get for the mass-murder" with all of the grace of a frost mage AoE farming gnolls to hit level 60 the fastest. Milestone XP is "You get XP from completing Appropriate Tasks, so do quests and tasks to level", whereas individual XP instead becomes "Whoops my brain regressed into all that time I spent in Runescape, WoW and FFXIV, time to de-populate this beach full of hobgoblins because they're the quickest way to level blue mage to 70 because I go from level 1 to 17 in a single kill and my warrior friend is keeping them all aggroed and murdered"
First off, IME individual experience doesn't lead to that sort of thing, but then none of us have that MMO background. Also, ideally xp are awarded for getting through encounters by means which can include things other than all-out murder e.g. negotiation, stealth, trickery, and so on; and are also awarded for getting past things you can't just kill e.g. traps, puzzles, and so forth.
Second off, if a player's main (or only, I've seen this) goal and focus of play is just to level up a character and get all those kool powerz, then the question "How can I easily get xp" simply changes to "How can I easily level up" and you're right back in the same boat. The only way around that is to somehow change that player's focus away from levelling and on to other aspects of the game - story, non-mechanical character development, the day-to-day run of play, whatever - with levelling seen as no more than an occasional pleasant side effect.
An extreme option would be to have a campaign where the characters never level up at all, but IMO that's probably overkill.
Third off, who gets to define what qualifies as "Appropriate Tasks"? If it's just the DM, that risks restricting not only what the players can have their characters do but to some extent how those things get done.
The other problem I kinda have with purely mission-based xp is that it highly encourages a commando style get in-get it done-get out MO from the party (the extreme of which is 3e-style scry-buff-teleport in-do it-teleport out), instead of encouraging/rewarding deeper exploration of the adventure site beyond just the fastest route in and out. This is perhaps the one redeeming feature of giving xp for treasure: it encourages exploration of the whole site if only to find any spare change lying around.