As a kitchen designer, I know both have their own pros and cons, and if you inform yourself well enough about how a kitchen comes together, how it is installed and will function, you will be better equipped to make a prefab or kit kitchen a success.
The key Is in planning. On paper first, some layouts lend themselves to kit kitchens better than others, between walls means you will need onsite modification and things such as fillers which bridge the gap between wall and cabinet, and ay need scribing to level uneven walls. If you have an l shape or open ended kitchen, then prefab is easy.
The main difference is making them fit with standard sized cabinets going up in increments of 50mm, whereas custom will fit into any sized space, and can accommodate restrictions from appliances, and varying depths etc.
The basic construction is the same, mostly.
A carcass of MR melamine will box out your cabinets, they can range from white melamine to aluminum boxed drawers, right up to timber fit outs and wire ware accessories. offered by both, however custom joiners offer more exclusive brands often with better warranties etc.
Prefab doors are ordered off standard sized carcasses, and ready made doors are bought in, limiting your colour and finish choice to whatever the kit provider offers. Whereas Custom is endless really.
To avoid prefab looking ho hum, and less considered, combine clever use of finish, colour or opt for an upgrade in your Benchtop, you can save on standard cabinets for example, and incorporate stone and timber in an island for example, or use your savings to include upgraded hardware, glazed overheads or designer lighting.
Prefabs when installed well can look high end with the right installation, and combination of finishes, just when you decide on them, don't compromise on design!
Consider colour and finish offered by prefab or kit companies and play around first with your layout options, then shop around to find a prefab supplier fit your needs!

Thank you, that really has made me a lot more confident in my decision to go prefab. We will be installing an I shape with island bench so fingers crossed it will all be easy sailing... Hopefully!
I figure if I can get the look minus the $ why not. Thanks again