Quick summary: if you're ok with getting the distance to the center of the green and not the precise yardage to the pin, then a GPS is a lot easier to use (You have to learn to hold the rangefinder steady). Higher handicaps are probably not affected by the minor inaccuracies of a GPS. Generally lasers are a bit cheaper than a GPS.
I posted this on another site about a month ago:
I recently purchased the Nikon 800 rangefinder new for $250 on eBay, and it's great to know the exact distance to the flag. Nikon sells an identical version of this model under the Callaway brand for $425 list.
Note that the 800 has two modes: First Target Priority and Distant Target Priority. FTP means that it will pick out the closest target of items grouped together. This is the mode you want for golf since you want to pick out the pin and not the trees behind the pin. DTP is more often used for hunting (i.e. get the deer, not the brush in front of the deer). Moreover, the default mode for the 800 is DTP, so golfers have to read the manual and switch it over (takes 5 seconds or less once you know how to do it). In the Callaway version I'm guessing FTP is the default mode.
I'm also familiar with mapping/surveying and even with WAAS a GPS reading can be +/- 2 to 3 meters. Here's one study: http://users.erols.com/dlwilson/gpswaas.htm.
That does not include the potential error when surveying the course. So if the actual survey was 3m off and your reading is 3m off you could potentially be off 6m or 6.5yards. That could be the difference between a short and a long putt. Finally, with GPS you're not getting the pin, just center of the green.
I've been able shoot pin placements up to 270 yards away and probably longer if I steady the Nikon on my golf bag, etc. So a GPS device is definitely easier to use (as long as you don't mind paying annual subscription fees and downloading courses before you play). With Nikon you do have to hold it steady (or use the scan mode which gives you a continuous read out). With scan mode you just have to learn to toss out the extraneous readings. For example, if I think I'm about 170 yds out, I'll toss out the 210 yd and 225 yd readings (those are the trees) and go with the 167 yd measurement.
Laser pros: no annual subscription, no course downloads, gets exact measurement to the pin, not the center of the green. I think a laser is more durable (for example, the Nikon is waterproof). I also think the laser uses less power (fewer battery changes) -- the GPS needs to be in pretty constant contact with multiple satellites.
GPS pros: easier to read, don't have to see the green to get a reading, gives you multiple readings at the same time (and a laser rangefinder may have difficulty seeing the back of the green, for example).
After using the Laser for a month, I guess I'd carry both if price were not an issue. Lasers are great for absolute precision to the pin (not the center of the green), but sometimes you can't see the green and need a layup distance. To fudge it, I'll shoot the distance to the 200 yd marker and then figure out my layup distance.
p.s. here's a program you download to your phone or PDA: www.starcaddy.com.
You buy maps one at a time.
p.p.s. the best way to determine the accuracy of your GPS is to team up with someone with a laser (since even yardage markers can be way off). I played a round with a friend who had a GPS. Sometimes he was right on and sometimes he was 5 - 10 yds off because he was getting Center-of-the-Green readings. Let's say the GPS says 170 to the center and it's a blue flag. Do you add 3, 5 or 10 yds?