Here is my two penneth on the great debate.
WARNING: This post contains serious window treatment nerdery, please look away now if you wish to continue believing in the existence of "a standard window".
This is not a question that has one answer. This is a question that as a designer I ask myself with every single window I see. I will attempt in the following post to shed some light on the different aspects to consider.
I do love curtains. They dress the window really beautifully, being by nature soft and flowing with curves forming the pleats they break up the rigid lines of architecture, immediately making a space more welcoming. Different types of curtains have their individual pros and cons (which I will go in to at a later date) but generally speaking curtains are the most dramatic and softening of window treatments. For me, curtains have to be full length. Short curtains really jar with me, so if there is a reason why the curtains cannot hang to the floor I will opt for a blind every time.
Roman blinds are the obvious alternative in these circumstances. They can be hung either inside or outside the window recess, and actually work very well if you are layering window treatments. You can happily put a Roman blind inside a recess with curtains on the outside, or a Roman on the outside of a recess with a Roller or Venetian blind inside.
Layering is particularly useful in circumstances where you need to achieve privacy or blackout in a room.
Roman blinds can be made from any fabric that is suitable for curtains, including voile or sheer materials as long as they have enough substance for the mechanism to work.
Here is an example of a layered window with a Venetian blind within the recess.
Venetian blinds are excellent practical blinds. They work really well as a privacy option as you can angle the slats to let in light whilst blocking the view into the room from the outside.
The faux wood options are also suitable for environments where damp is an issue as they can easily be wiped clean and will not hold the moisture and split or warp like real wood can.
Roller blinds can sometimes be more practical than Romans, for example if you have a window where the blind must fit within the recess but you are concerned about getting enough natural light into the room. A Roman Blind in the fully "up" position is still going to cover around 25cm of your window, so if you cannot afford to lose that light, opt for a Roller.
Bespoke Roller blinds can be rolled one of two ways. What is known as standard roll is where the material rolls off the back of the roller, meaning that the fabric comes down closer to the glass. If you are using a Roller Blind for blackout purposes, this will get you a much better result than reverse rolled as it cuts down the light bleed around the edges of the cloth considerably.
Reverse rolled involves the fabric rolling off the front of the barrel. This is preferable if you need to avoid the blind clashing with a handle on a window, or if you are having a blind made with a fabric laminated especially for you as otherwise you will see the plain colour of the laminated cloth across the top of your blind instead of your chosen fabric.
The other types of blinds that exist are more specific to a style or function.
Plantation shutters have proven to be gaining in popularity in the West Country in recent years. Shutters work really well in coastal properties and in modern schemes that are incorporating a lot of white.
Panel blinds are inspired by Japanese sliding panels. As there are various eastern influences pressing forward in current design trends it is possible that we will be seeing a lot more of these adaptable systems. I particularly like using them in rooms that need to change function as they can easily be used as room dividers that can simply stack where appropriate, making the space extremely flexible.
Vertical blinds are a firm favourite in office environments to avoid glare from the sun without losing natural light.
There are times when pleated blinds are the only option. These can be shaped and tensioned, so when faced with a triangular window, or a window where the blind has to open from the bottom up instead of from the top down, this is when I look to pleated blinds.
I use all of these options depending on the needs of the space. Hopefully having read this post you too will have the confidence to consider all of these options part of your tool kit when you next have a window to furnish.