I agree what many are saying here and I think you've just got to try for yourself to a degree.
I.e. Buy the main plants, the ones that are most important to your display from the supplier B, paying through the nose on 2 or 3 plants that are very important to your tank layout. The chances are you'll be so happy with the product you'll soon forget the expense.
Then spend the same or even half as much with supplier A and get lots of plants to look through and perhaps not use them all but benefit from choice on both quality and product will prove useful to you. This gives you the chance when ordering to choose a few unusual items that you've not seen in the shops perhaps, and it allows you to pick and choose without breaking the bank.
Each supplier has it's merit in my book, and an online retailer doesn't need to make as much money and also doesn't have customers viewing and criticising poor plants, so offering the cheap stuff with people being less offended if they get one or two dodgy specimens isn't a huge risk.
Whereas the shop with all the overheads who need to make money to stay there, they have to find the balance of great plants(expensive, but customers see that they do get what they're paying for and appreciate it) or taking a risk on lesser quality plants but still needing to make a decent profit so unable to slash prices to compete with some online companies. This means a middle of the road price and some good plants, some not so good. If I had a shop Id be leaning toward the quality expensive plants for sure.
At the end of the day, for a business, the problem is 'you can't please all the people all the time'
I've bought at both ends of the scale and would use both suppliers again when I need to.
One thing to remember here is 'aquascapers' make up a small amount of folk in this hobby. Most people setting up a planted tank for their living room really don't wish to start re planting in a month or two. So the other way to look at it is why spend all that money on a tank and hardware to decorate it with poor plants that need major care and time to develop, when spending alot more (but still nothing in the scheme of things) can offer a big finished display that makes you stand back and just smile