To the best of my knowledge, more is more in this game. I pumped my attack a lot to ensure higher chances of success on Fights (or heists as well I think).
Defense is good I think for repelling attacks but I've found this to be suboptimal to invest in since there will always be that guy who's a hundred levels above you. Such a player will just succeed in attacking you no matter how much defense you have simply because he/she would have had a much larger inventory of items, mafia or stats.
So my only advice is just get your mafia size as big as possible and level up quick by playing often (if you're so inclined) so that you can be the one preying on lower level starters . Also, when buying property, buy in bulk because the prices of property go up after every single transaction (regardless of that single transaction's size).
Ultimately, just have fun, it's meant to be a semi-social game. Whatever happens in Mafia Wars has no bearing whatsoever on your physical life, so just enjoy the game, and don't worry about losing money. It's very easy to earn it back with property investments.