A couple of hints on ridge vents:
1) In snowless or no-snow regions Do Not continue them out the ridge to the overhanging eave portion - it weakens the plywood sheathing connection and can cause your roof sheathing to peel back in this area in heavy winds. You do not need them here anyway - only the enclosed attic portion needs them. In areas with roof snow load common in the winter, continue them to the end of the roof ridge but do NOT cut away the sheathing below them in the overhang area. Explanation why in 2) below.
2) Be sure the ones you are getting have a labyrinth cross-section or exterior baffle that will trap blowing rain and snow. Snow especially (although storm-driven rain can too, especially in thunderstorm and hurricane prone areas) blows up along the roof right in under the ridge vent overhang, so if it is not designed to trap that snow and water and direct the water back onto the roof it will end up in your attic insulation and ceiling. I don't have enough fingers and toes to even begin to count the number of times I have seen cheap direct shot ridge vents or the "steel wool" appearing mesh ones let masses of blown snow into an attic. In one there was over a foot of snow standing on the insulation - and of course melting down into the house below.
3) In the case of a house with good attic insulation, there is not a lot of heat in the attic, so the ridge vent does not melt the snow and can have winter-long snow cover. This is VERY common where you have about 8 inches or more of snow cover for extended periods of time. In my area, you can tell the moderately to well-insulated attics by their ridge vents being fully snow covered and useless all winter long. This means, if you have closed off your gable vents, that household moisture coming into the attic (and there will ALWAYS be moisture) is trapped inside, and builds up as frost on the top of (or within) the insulation and on the underside of the roof. I have seen up to 6 inches of frost buildup from this.
Therefore, for this reason I use three things in my designs for heavy snow areas like where I live:
1) run the ridge vent full roof length and leave the ends open, so the two ends are exposed to open air and not buried in snow pack, allowing end venting. However, do NOT cut the sheathing gap slot in the gable overhang section - just apply the ridge vent over the sheathing and ice and water shield, to provide a vent area in an area where the snow commonly metls off faster. This works only with vents tht have a continuous opening down their length - with the cross-baffled type with no central opening this does no good.
2) leave the gable vents open. This does not really hurt the eave-to-ridge venting, as the most significant part of that airflow occurs in the space between the joists right under the roof, and the gable-to-gable or eave to gable air passes underneath that zone. Also, if moist or hot air escapes through the gable vents rather than the ridge vent, it is still gone. Also, gable vents only work significantly when there is at least a small breeze or pressure differential between ends of the house, in which case natural drafting will be sucking much larger amounts of air than usual through the ridge vents at the same time anyway.
3) I require an ice and water shield strip (typically about 8-12 inches wide), covering (and stapled to) the edge of the sheathing vent slot at the ridge, downslope past the edge of the ridge vent, and exiting on TOP of the shingles at the downslope edge of the vent. What this does is capture any water from snow and rain dropping within the vent and keep it from getting on the exposed shething at the slot or under the shingles, and carry it away back to the roof surface. Some ridge vents have a solid aluminum or plastic bottom base piece that does this so the ice and water shield is not needed in that case.
I have not seen Cobra vents but from the pictures I found, it looks like they are wide open to snow and rain infiltration like the mesh ones - although maybe the Snow Country version is better - could not find a closeup of the design. Vent Sure I have used and like - it has good baffling and moisture shield to prevent water from getting down into the attic.