End mill choice

I have an old nm-135 with a 3500rpm spindle. I have been running the following :

1/4 3fl carbide end mill 18ipm .07 doc 3500rpm
3/8 3fl carbide end mill 18ipm .08 doc 3500rpm

I need to get new endmills and wondering if I should change. A lot of people suggest using just hss or cobalt. I don't want to slow my cuts down, quite the opposite. I do almost exclusive slot cutting in 6061. Any suggestions would be appreciated. Also if you really like a certain brand..

Thanks - Keith