I do not believe Hitler would have attacked France first. A big part of his army was still unexperienced, the German tanks division didn't prove themselves in battle yet, neither did Luftwaffe. To send it all against the strongest enemy would have been too risky even for Hitler. Not to mention the fact that Poland would have immediately declared war on Germany, forcing Hitler to leave a significant part of his army on German eastern border, probably bigger than forces left at western border in September 1939. Why bigger? Because Polish-German border was longer and it wasn't so well fortified. And Poles would have been more active than the French were IOTL. Polish Army was weaker and most certainly wouldn't have gone for Berlin, but is still could make some painful blows, especially in Upper Silesia