I hunt with a Swarovski 4-12x50 TDS on a 5.5 lb. 6.5 STW and have had zero problems. Swarovski has always kept Kahles a few steps behind them for marketing reasons....until recently that is. Kahles is now completely off the leash at their request and have substantially closed the gap. I am sure you have noticed this over the past couple of years. Their new CSX illuminated scopes offer superior technology when compared to the Swarovski illuminated scopes. They are almost becoming fierce competitors...which is good for us because we just keep getting better and better products. These two companies do not share technology, rep groups, staff, etc. So all that being said...I would still go Swarovski but keep my eye of Kahles in the future.
If price was a factor I would go with out beer for a while until I had enough to get the Swarovski.
There is a difference but mainly just in extreme low light. If you go Euro you definitely pay for it. 4200's, VX-III's and Conquest are all above average and can easily get the job done but if you want the absolute best money can buy then it would be a Swarovski.