Vertical and roller blinds solve different problems. The simplest way to choose is to look at the window: rollers are made for standard openings, verticals for the big ones.
The basic difference
A roller blind is a single sheet of fabric on a tube that rolls up and down. A vertical blind hangs as a row of fabric louvres (vanes) that rotate to angle the light and draw to one side to clear the window, more like a curtain of slats.
Window size and doors
This is the key difference. A roller has a practical maximum width, beyond which the tube sags, so very wide windows need two or more. A vertical spans wide and tall openings easily and, crucially, draws aside - which makes it the standard choice for patio doors, bi-fold doors, bay windows and conservatories, where you need to walk through or where the glazing is large. For a standard window, a roller is neater.
Light control
A vertical tilts its louvres, so you can let in angled daylight while keeping privacy and cutting glare, without drawing the blind back - useful in a south-facing room or a home office. A roller only moves up and down, setting the light level by how far you lower it and by the fabric, which is simpler but less adjustable.
Look and cost
Rollers look neat, flat and modern and suit small and standard windows; verticals have a more functional, office-and-conservatory character but handle scale that a roller cannot. On price, both are among the more affordable made-to-measure styles, with cost driven mainly by the size of the window.
Which to choose
Choose a roller for standard and small windows, kitchens, bathrooms and a clean modern look. Choose a vertical for patio and bi-fold doors, bay windows, conservatories and any wide or tall opening where you need to draw the blind aside and tilt for light.