As far as getting a whiff of the resin every now and then, there can be a number of reasons. Facial hair, straps on the mask not tight enough, old filters, etc. Styrene isn't THAT extreme, I believe the regulations where I am state that if you work with it for more than 8 hours a day 5 times a week then you should be ensuring exposure less than 50ppm. 50-100ppm can be described as having a detectable odour when you walk into the work space, but not so strong that it irritates the nose, throat or eyes. This is a bit subjective because some people will feel irritation at 100ppm, some won't feel any irritation until at least 350ppm. In any case, getting a whiff of the stuff every now and then is harmless. It takes prolonged exposure to do any damage, and if you can take your mask off at the end of the day and still smell then styrene then you likely haven't exceeded the maximum exposure limit (your sense of smell dims with exposure, so people who have exceeded exposure will no longer be able to smell the vapour). If you feel any irritation at all, or if it feels like you have a blocked nose, or you feel vertigo (a bit dizzy like you've had some beers), then you have exceeded exposure limits and you should take measures to ensure it doesn't keep happening. 

The alternative is an air-fed mask. In theory these are better, but to be honest I have my concerns about the oil vapour coming from the compressor. I prefer to wear a respirator and ensure plenty of ventilation. 

So would you say this mask is sufficient? Is there anything better I could be using. Thanks 

Can't tell if yours is good by looking at the picture, but all it needs is organic vapour cartridges rather than the simple dust gauze filters. I believe they are generally called 'A1' filters while the dust filters are called 'P2', though that may be an Australian thing. 

I can sometimes get a wiff of the resin with the mask on. And more than anything worry about long term effects.