[4e] What is worse?

[Guinea Anubis]

Looking to put together an encounter for my PCs that are all level 5 or 6. I was going to add in some Stirge. I was wondering what you all think would be more challenging to fight ten level 1 Stirge or five level 5 Stirge?

[Sitzkrieg]

Definitely five advanced Stirge. With an AC of 16 and only 5 hp, a fighter can just use great cleave and take out as many has he/she can reach per round. They'll also be vulnerable to low-level mass effect spells like Sleep.
EDIT:(I just realized you meant 4E, which I do not know. Probably the same logic applies though, I wouldn't expect a large number of easy enemies to pose much threat)

[Aron Times]

The five level 5 stirges are going to be the harder challenge. The level 1 stirges will have difficulty hitting the party and will be very easy to hit.

[Inyssius Tor]

EDIT:(I just realized you meant 4E, which I do not know. Probably the same logic applies though, I wouldn't expect a large number of easy enemies to pose much threat)

The same logic does indeed apply, yes. They'll practically be two-hit minions except with much lower defenses and much weaker attacks.

[Yakk]

Five even-level stirges isn't a serious challenge. It might drain a healing surge or two.

10 level minus 4 stirges is going to be a tad easier. L-4 is pushing the edges of what you should do in a single monster, especially at low levels.