Inspect your prospective purchase for workmanship. If you are satisfied on all counts in that respect, the choice is up to you. Again, you will find a larger one of more use, as you will wish to furnish it with garden chairs, a table, and other items to make outdoor living more comfortable.
Uses. Many people buy a summerhouse intending it to double up as a shed, but if you want to use it properly, i.e. as a leisure extension to the home, this really is not practical, as who wants to take afternoon tea surrounded by all the junk, machinery and equipment you shove into your garden shed? It looks terrible from the outside, too, as summerhouses in general have much more glazing than ordinary sheds, so everything is on view. Some inexpensive summerhouses are often open-fronted anyway, so you could not use them for the permanent storage of anything that mattered. Most summer-houses are more expensive than the equivalent cubic capacity in a garden shed, so it is a dear way of buying storage space.
Do have a paved area outside the summerhouse so you can spill out on to this without damaging the surrounding garden. This is a good place to construct some form of barbecue.