First lets look at materials and styles. Most common materials are PVC vertical blinds (vinyl), fabric, and fabric insert. PVC vinyl technology has improved dramatically. You can get a rich textured look or smooth soft look. Both are available in many colors. The back side on all is usually a shade of white. Vinyl vanes are mostly curved so when rotated to a closed position, they close tightly. Vinyl's are easy to clean and hold up well in high traffic areas.
Fabric or fabric free hang are an excellent choice if you want light filtering. They don't close as tightly as vinyl when rotated shut. They also come with weights at the bottom of each vane. Some companies give you the option of chains attached to each weight.
This is supposed to hold the vanes straight and keep them from flopping around on a windy day. I personally don't like them. They are tedious to install, break easily, and pose a hazard to children and pets. If you don't want the weights or they are not offered, then ask if the weights are either sewn or glued in. If they are not, they will slip out and become a pain to deal with.
An alternative to free hanging fabric is fabric insert into a vinyl vane. This allows for the softer look of fabric, combined with the more uniformed look of vinyl. They won't have weights at the bottom and will close much tighter. The draw back is occasionally the fabric comes out of the insert. My wife won't have them anymore because of this. She got tired of putting them back in. My experience is, the thinner the fabric, the easier they slip out. If you like this look, I suggest getting a heavier fabric insert.
Moving on to the valances, most will come with a standard valance. You might have seen them with clear plastic corners at each end. Depending on the manufacturer, they may have several upgrades to choose from. One upgrade is commonly referred to as an executive or wrap around valance.
The balance corners are rounded providing for a softer, uninterrupted look. Another option is a double valance. This is more difficult to install, but I like using it on wider windows. For example, a patio door 3-4 panes of glass is 10-15 foot wide. A standard 3-1/2 inch high valance looks to skinny. A 6-7 inch high double valance looks more proportional.
Headrails are to verticals as curtain rods are to curtains; but not all headrails are created equal. One thing to consider; if an upgrade is available, take it. It will be worth the money. You have gone through all the work of creating a beautiful look, you want to make sure it functions correctly and for a long time. You don't want it jamming up and not rotating correctly, or controls coming off. Price does dictate quality when it comes to headrails.
Also ask if it has interchangeable carriers. The carrier is the piece of plastic that the vanes clip into. If the plastic breaks, the vanes fall out. With interchangeable carriers, you simply take out the broken one and insert a new one, done. If it doesn't have that feature, you have to send the headrail back for repair. Lastly, check to see if it has self aligning vanes. Have you ever seen a vertical where all the vanes are not facing the same way? All of them rotate except a couple. This is another trip to the repair shop.
However, if you have self aligning or self correcting vanes, you simply rotate them back in the opposite direction completely and back again and they we self align and look uniform again. All manufacturers come with child safety instructions. I suggest you read these carefully. Some places offer cordless controls. In this case, a wand is attached to the headrail to control both the draw and rotation of the vanes, eliminating any cords or chains.
In conclusion, a bit more investment here will be worth it. Discount blinds will have the lower end headrail and thinner vanes which are very flimsy. There is something for all needs from patio doors to living rooms, to office complexes, and rental units. One last tip, keep 6-8 inches from heat vents. Good luck and I hope this helps.