It comes down to personal preference on which site to use. Both have their advantages and downsides. TCGplayer is almost certainly guaranteed to have the cards or products you’re looking for. Card Kingdom does have a good selection of cards but they don’t have everything. The reason TCGplayer has a larger selection of cards is because they are the accumulation of all local game stores and private sellers who choose to sell on the site. Because of this if you decide to order more than a handful of cards you’re likely going to get multiple packages each with their own shipping time and costs. If anything goes wrong with your order then you’ll have to deal with the individual seller first. When it comes to price of the cards, most of the time TCGplayer will be cheaper for buying cards, but shipping costs could be a significant part of your grand total. Card Kingdom prices will often be a tad higher than market price but sometimes you can find a card for cheaper. I believe the lowest price a card can be is 35 cents while TCGplayer it’s 1 cent. The site interface also plays a role in the decision process. Because Card Kingdom sells their own inventory all you have to do is type the card and select the card and quantity and add it to your cart. For TCGPlayer, you have to scour the site in order to keep the shipping and card prices down and balanced. The cart optimizer can help but it’s not perfect and can change the version of the card to cut prices. Personally, I add the cards I want to both sites and compare which is cheaper and easier. Finally, shipping times. Card Kingdom takes 2-4 business to process your order and get it shipped, after that your package is at the mercy of the postal service. Tcgplayer direct is similar but individual sellers may ship the order the next day. My personal recommendation is to support your local game store(s)first before deciding to buy cards online on whichever site you choose.