Anywho, I wouldn't touch the "every X milliseconds" condition with a ten foot long clown pole.
WHAT?
THEN YOU ARE
DOING
IT
WRONG
Allow me to inform you of a little thing called framerate independence, forum member Oko. If you use Always, then what happens if you run the game on a very slow or very fast computer? The game will run slower or faster, that's what. People on slow machines will have an easier game, and people on fast machines will have a much harder game, because stuff is spawning at like, twice the speed or more.
When you use Every X Milliseconds, you GUARANTEE that it will spawn at the same rate on all computers. REMEMBER THIS WELL. SEAR IT INTO YOUR BRAIN WITH A HOT IRON. YOU HAD BETTER START USING EVERY X MILLISECONDS OR I WILL HUNT YOU DOWN AND CRUSH YOU.
Look up more info about this on the wiki.