I worked for many years as a window rep, and have heard that a space over 5/8″ can get a convection current inside the insulated glass, making it less energy efficient.
The distance between panes is a function of insulation. Insulation works by trapping air and causing slower heat/cold transfer. A larger space is better you can also have the space filled with low “E” Argon gas or a reflective UV membrane.
Choose a reputable manufacturer and do some research online the time invested now will be cost effective later.