I use both. Psyshop seems to get new stuff quite a bit before Saiko gets them. This is Psyshop's main advantage that I can see.
Shipping from both is fast to the US. Shipping is similar in cost for both companies. CD cost is similar as well.
Saiko has a great "add to wantlist" feature. Saiko is VERY friendly in any communications I have had with them. Saiko also offers a freebie CD (choose from a list of many options) when you spend $100 or more. Saiko seems more "flexible" to special circumstances than Psyshop.
Psyshop could work on their communication, as it is severely lacking, sometimes even rude.
Both companies will toss in stickers, magazines, etc.