which disk file to choose or which can give me better output

Hi Friends,

I am installing mail server for around 500 users and will be virtualizing on proxmox. Since its a mail servet iops will be higher hence which disk file is suggested? Vmdk or raw or the other one? Also i m choosing kvm 64 bit and using ide as disk type. My server conf is as below

Hp ml 150 g6 with 16 gb ram. 2.13 xeon processor with 4 core and 250 gb sata in rad 1 and 1 TBx 2SATA hardisk in raid 1 again.

I have installed proxmox on 250 gb harddisk and will be using 1 TB disk for images.

Please advise.