In my experience, the more competitive the league is the quicker top goalies get drafted. Good managers know that elite goalies win championships in formats like this. In my leagues, I expect almost every manager to draft a goalie in either round 1 or 2, but likely not both. So I would count on 12-14 goalies being gone by the end of the second round. Look at your list of goalies and decide if your 15th ranked goalie is good enough to be your go-to starter all year - unlikely. You want to secure a top 10 goalie and this should be feasible unless you pick very high in the draft, in which case the risk is mitigated by you getting a 120 point player with your first pick.
So I would say:
Drafting early - Take one of the big 3. I doubt one slips past 4 but I would take one of them at 5 and every other spot below that, too, if for some reason you are drafting with people who like losing!
Drafting in the middle - This is where I would love to secure a top goalie like Luongo, Brodeur or Nabokov. You will still get a great forward when the snake gets back to you, ideally a winger.
Drafting late - You can often get away with taking a forward with your first rounder (or Mike Green, if he's there) because you know your next pick is coming soon. It will come down to how many goalies have been taken and how many you expect to go between your 1st and 2nd rounders - judge accordingly.
It boils down to something like this:
You want to make sure you get a top tier goalie. If there's a big early run on goalies and only one or two of your top tier guys are left, take one. It's too important to try to be clever and buck the trend, and you'll end up scrambling later. Taking two goalies back to back, though, is probably overkill because you can usually get a solid #2 in the 4th or 5th round and ensure both solid netminding and offense. It can be hard to make up the difference in scoring if you don't pick a forward until the third round, especially if it's the end of the third round in a 14 team league.
General draft strategies for me consist of awareness and flexibility. Awareness meaning you know who've you taken at what position, and what kind of performance they're likely to bring to the table. You know that if you've drafted two solid left wingers already then your third one can be a high risk, high reward type. You know that if your #1 right wing is Gaborik (heaven forbid) then you had better find two steady performers to back him up. You know that if you're weak on goal scoring compared to playmaking through the first half then you need to adjust your list to look for goals in the second half, or vice versa. This kind of thing.
Flexibility is an offshoot of awareness and means that you can't rely on hard and fast rules. Sometimes (always?) drafts go differently than you expect. Even if you know you have the first half dozen rounds mapped out, things change. Don't be afraid to adapt your strategy if, say, a stud center falls to you even if you were planning on taking a winger at that spot.
In my opinion, the LW and RW pool become rather similar by the time you get to the 30th ranked players on each side. There may be more of a difference at the top as I suspect there are a few more elite LWs than RWs. I will be focusing on both these positions with a slight edge to the right wingers.
Adding a forward slot doesn't necessarily make wingers more valuable, since this slot can be filled by a centerman. In fact I would say if anything it makes centers slightly more valuable since you can get away with starting 3 of them each night. Defensemen do become a bit less important when you only start 3 and I would stick to the following rule: if you can't draft an elite dman early on then wait a few rounds because defensemen level off quickly and become very similar after the studs are gone.
As for +/-, in your example you compared an 80 point, +20 player with a 90 point, +10 player. Keep in mind then that the average 90 point guy will be better than the 80 pointer in not just P but also G and/or A, PPP, and will have at least a head start on SHP and GWG. Take the better point producer unless the difference in predicted +/- is astronomical and the points are similar - they will contribute in more areas.
20 team H2H keeper. Keep 10. Now drafting...
Start 2C, 2L, 2R, 2W, 2F, 4D, 1G daily
G,A,+/-,PIM,PPP,SHP,GWG,SOG and W,GAA,SV%,SO
C: Getzlaf, Zetterberg, Mueller, Gerbe
L: Parise, Elias, Prust, Pouliot
R: Gaborik, Briere, Clarkson, Brouwer
D: Weber, Yandle, E. Johnson, Zidlicky
G: Howard, Gustavsson, Fasth
Farm: Toffoli, Tatar, Lehner, Josi