If using a template guide bushing, a 1/2 in spiral with a half inch shank should last quite a while. Masonite, MDF, particleboard, etc. can be hard on bits, but a good quality solid carbide spiral should serve you well.
Some examples:
https://holbren.com/home.php?cPath=22_98&osCsid=39da35aa57616b79c1bc00195886eb42
http://pricecutter.com/product.asp_Q_pn_E_P11-1501
http://www.routerbits.com/cgi-routerbits/sr.cgi?1132186894_23797+9
If you are going to follow a template using a bearing (not recommended for doing a lot of pieces in my opinion) a quality flush trim bit with some shear angle will work ok. The fine dust from masonite might gum up your bearing on that bit however.
Hope this helps,
Some examples:
https://holbren.com/home.php?cPath=22_98&osCsid=39da35aa57616b79c1bc00195886eb42
http://pricecutter.com/product.asp_Q_pn_E_P11-1501
http://www.routerbits.com/cgi-routerbits/sr.cgi?1132186894_23797+9
If you are going to follow a template using a bearing (not recommended for doing a lot of pieces in my opinion) a quality flush trim bit with some shear angle will work ok. The fine dust from masonite might gum up your bearing on that bit however.
Hope this helps,