There's a reason you can change your 2nd prof, try both. Although the only use of R/N I see is toucher, which is not that effective in either modes. [skill=text]Mending Touch[/skill] is used a lot on R/Mo, and for PvE you could take hard resurrection skills.
You can't really say that one is better than the other, if your focus is goingto be on the Ranger it doesn't matter as you can change secondary as often as you like.
Ranger/Monk could provide you with skills to keep yourself clean from conditions, for instance Mending Touch, getting rid of that blind can be useful often.
Ranger/Necromancer could provide you with the all too popular touch skills, a few other useful secondary skills, for instance the Plauge skill series to get rid of conditions from yourself.
So none is better than the other, it's simply which fits better with what you want to do.
I play Ranger and I haven't seen any uses for R/N. But I REALLY hate touch skills. What's the point of being a Ranger armed with a bow if you're just going to get up in their face? It's much more fun to stay as far away as you can and turn them into pincushions.
best secondary for a ranger is probably monk, just makes a nice combo as you can use either a rez skill like rebirth or a heal with condition removal like mending touch
necro best secondary depends what type of bar you wish to run...monk and ritualist are probably my favourites depending on what type of necro it is!
you can use any weapon no matter what profession you choose...might not be as effective as a warrior using a sword but you could still use it..for best effect have warrior as 2nd profession and put some points into swordsmanship rather then marksmanship
people generally prefer hammer and sometimes axes over swords as a melee weapon alternative to the bow
also scythes work sometimes, rarely used in PvE but does occasionally work depends on what you want to do!
R/N can be an effective PvE'er if you go down the curses line ... for sinstance MoP + Barrage can be very effective.
Though R/Mo is very common, there are others out there... R/E is very effective for a high damage fire ranger... or you could go with a R/Rt and use splinter weapon...
when i first started out as a ranger i was a r/mo, but rarely used my monk skills, i went through a few secondry professions... r/mo, r/a,r/n,r/w ...currently im using a r/e as a glass arrow fire ranger for spiking and a r/ whatever for the popular BHA
for necros the top 2ndaries are probably monk or ritualist as posted in an above post somewere :P
if you are a minion master necro then monk is good to keep them alive (kareis healing circle, one non-necro option for keeping your minion army alive)
flesh of my flesh is a nice option if you have ritualist as your necros 2ndary profession!
I found if you are running glass arrow it is best to be r/rt with splinter weapon included in that bar for wide spread bleeding etc.
all 2ndary professions have up and down points!
/monk is popular just because the skills of rangers and monk styles can work very well together!
/ele if you would like to do fire / water / earth / air based damage with your arrows
etc...all are good some are just alot better then others!