I've bought two kitchens this year:
1 - Small galley kitchen for annexe.
Purchased units from DIY Kitchens and had my trusted local carpenter/fitter install for me. Bought by own appliances, organised plumber, tiler and electrician myself.
Units are fine, pleased with the overall finish for what it is. Wish I had made a slightly different decision about the layout. I bought what I thought would work and it's fine but could be better in hindsight.
2 - Main kitchen & scullery in house.
Looked around for a very long time, had several quotes but went with local independent kitchen supplier who had excellent customer feedback and gave really help input into the overall kitchen design. Attention to detail was incredible from lines, symmetry to function and placement of everything to make the kitchen workable.
Units are German brand (Beckermann) and are superb quality. They are also custom made to exactly the sizes we need where necessary so everything fits just perfectly. Kitchen company has supplied and fitted everything - units, worktops, lighting, appliances, tiling, glass splashbacks etc. Organising the relevant trades (plasterer, electrician, plumber, tiler etc.) has all been down to them, no stress for us.
Any issues are their problem - quartz worktop fitter scratched a couple of unit drawer fronts and these are being replaced within hassle. Had I opted to have this done separately I can see this would have been a challenge to get sorted out.
Quality and finish is fantastic. Units are much better quality than the DIY ones and feel like they will outlast them considerably. Layout is perfect, so workable and looks amazing.
Ultimately your budget will determine what approach you take but if you can afford it I would go with a local independent with a good reputation every time.