Is there a "correct" forum to file in or is this at the discretion of the complaintant ?
It is at the discretion of the complainant.
If the language of the registration agreement is something other than English, you may want to use WIPO. NAF will categorically refuse to conduct proceedings in certain languages.
The filing fee at WIPO is $1500. At NAF it is $1350. If you want to file additional statements, WIPO will take them for free, notify the panelists, and the panelist(s) will decide whether they want to see them. NAF will charge an additional $400.
2- How is it determined whether there is a 1 person arbitraitor or a 3 person panel? Does the complaintant have that perogative?
If the Complainant wants a three member panel, then the Complainant can pay the full three member panel fee up front. If the Respondent wants a three member panel, and the Complainant has not paid for one, then the Respondent pays half the three member panel fee. At WIPO, a three member panel is $4000 - so if the Respondent requests one, they have to chase down the Complainant for another $500 (since the Complainant only paid $1500 to file). That sometimes causes a stalement. The NAF wisely sets the three-member panel fee at twice the single member panel fee.
I can conceive of no circumstances in which it is desirable for the complainant to select and pay for a three member panel.
Since the UDRP requires the three member panel option, all UDRP providers have a procedure for it.
The upshot is:
Language other than English - use WIPO.
Don't expect a Response - use NAF.
WIPO and NAF merely administer the cases. There is considerable overlap in the roster of panelists, so you can get some of the same panelists at either place.
NAF is generally better at keeping these things on the mandated schedule, and processing them promptly. WIPO tends to be more clueful about working out a balanced procedure in complex cases.
Having been represented both complainants and respondents in dozens of proceedings at both,there's not a huge world of difference, except under the circumstances I've noted above, and with NAF having an edge on speed.