Gmail spam filtering is actually poor, in my experience, because it's so aggressive. Around 30-40% of the mail in my spam folder is legitimate email, and the ratio is sometimes 90%. It would be even worse if I didn't have filters to tag some mail. (Gmail gives you the message it was not sent to spam because filter.)

Outlook.com's spam filtering is at least as good as Gmail's now, probably better. The only significant difference is that it's much less aggressive. Outlook.com will let two or three spams through where Gmail will put four or five legitimate emails in the spam box.

Since most people probably don't check their Gmail spam boxes regularly, Gmail's spam blocking is actually more dangerous.