So long as your tanking comes from a merc, I'd lean towards a Monk based simply on Pulling ability.
Ranger pulling is very much handy a lot of the time, but a zone plagued with root/snare immune mobs can make it difficult or completely impractical. In a dangerous zone, a pull involving potential run-immune adds leaves very little room for error.
Also, while an ability like Fluster/Blusterbolt can function in both pulling and crowd control, any type of root crowd control can be a complete mess if you use a tank merc. They simply do not play well with the abilities, and nothing is worse than a frantic few seconds of punting mobs away only to see the Tank stupidly charge them before you can rein it in.
There is also no comparing means for dropping aggro, Ranger fade can't be cast on the move, requires luck/skill to successfully use against see invis, and has a pretty high reuse of over 10 minutes even with hastened AAs. Meanwhile, a monk's FD needs little explanation on how versatile and amazing an ability it can be.
Tanking I may give Ranger a nod on. Monks are no slouches at avoiding damage, but I believe a Ranger is a little more all around equipped to step in and play tank when necessary. (Ranger tanking not recommended in anything more than mildly challenging content, in my opinion.)
DPS? I'm fairly Monk ignorant on this, but I know a Ranger takes a good amount of button mashing to really pull off a good burn. I could be completely wrong but a Monk may require a good bit less micro-management in this department.
As far as something like Headshot, it *is* pretty awesome, but it is one of oh-so-many Ranger AA lines to be worked on. If you are prioritizing Headshot XP as one of your primary ways of XPing it can be very much worthwhile. However if it's something you tire of, or you spend a lot of time doing other things, the investment of AA required for headshotting may be a waste.
That being said, I'm probably as happy now with headshot soloing now as I have been since they nerfed the ability to AE headshot. The Kaesora Library instances in SoD are right up my alley for easy, laid back, yet very good xp soloing.
Finally, AA investment could also be a consideration. The extra all around utility a Ranger brings to the table also requires that much more AA investment to really make it something special. It took a ton of AAs before I really felt like my Ranger was getting somewhere in all the various roles it can play.
Despite that long post, I think this is all moot if you just plain found a Ranger more fun. That basically counts for more than anything.
Sorry if this post rambles on in places. I waited to post and added to it, while perhaps a little too tired.
Edited, Nov 30th 2011 8:09am by Saeel