Speaking from experience here. I have the newer 1.5, 118 BHP (16 plate) estate and the wife has the older 1.6, 115 BHP (61 plate) estate. The newer 1.5 is so much better to drive, it's more responsive lower down the rev range and just feels nippier to drive. It's by no means fast though!
To the op, it seems like you could be looking at the wrong car and it may be worthwhile re-evaluating your requirements. Perhaps take the 1.5 for a test drive, but if it's more poke that you require it may be time to look at alternatives. I certainly wouldn't be buying a relatively new car that could potentially still have manufacturers warranty left on it and then re-mapping it.