What do you mean by faster? Faster to open/close the app, faster to open an email, faster to receive sent emails, etc.?
It's hard to say which uses less battery. Yes, different apps can use a different amount of battery, but for email apps, a big factor is how the accounts are set up, and what type of accounts they are.
If you have a POP account set up to check email every 5 minutes, that's generally going to consume more battery that an IMAP account. However, if you set it to only check every 1-2 hours, it's most likely going to consume less battery than an IMAP account.
My primary account is my Gmail, and the Gmail app works fine for my purposes. I tried K9, but it wouldn't work with my Yahoo account over IMAP. I'm currently using MailDroid for my other email addresses, partially because it works with my Yahoo account over IMAP, and also because I liked it better than K9. I can be a little bit laggy at times (opening the app itself), but not enough that it really bothers me. I've changed it to cache emails as they come in, and email open almost immediately. I have 4-5 accounts set-up via IMAP, 3 via POP, and MailDroid uses very little battery/CPU.