I considered a twin for awhile, specifically the Cessna 340 or one of its close sisters. With an avionics overhaul it could be had for potentially half the cost of the singles I'm looking at. However there were a few things that lead me away after about a month of consideration:
1. Complexity - It's simply more plane to manage, and unless I am a professional pilot I dont want my own inexperience getting the best of me.
2. Fuel burn - I fly mostly alone, so I kinda felt silly and wasteful flying a twin everywhere I went. I'm not particularly a greenie or anything, but the thought of using 2 engines just to sit on my butt just felt "wrong" more or less
3. Emergencies - I side with the camp that says 2 engines = 2 things to go wrong. I've never flown a twin of course, but from what I've seen and read, flying at max power on a single engine during takeoff can make for a very interesting experience. Personally I think I'd rather just plan on an emergency landing in my single and look for some bushes instead of mitigating the laws of physics in a failing twin. I'll explain more if you dont know what I know about twins, but lets just say it's not as easy as going full throttle and making a go around to come back. Hopefully some twin pilots can back me up.
Thats about it really. I do have family and would love the option to fly to the Bahamas safely and confidently in a twin, plus having all the cargo space necessary to achieve this. The other biggie is the range factor. I will be flying in excess of 1000mi once to twice per month. Not having to make as many pitstops, or any at all would sure be nice in something like the 340.
One thing I'll note is that your fuel consumption is probably off. Sounds like you're comparing dry weight to dry weight and assuming any dollar saved in your twin is a dollar towards your luxury single. Dont forget resale value either. I dont think the market for twins is anywhere close to that of singles. Which do you think is going to be easier to sell in 10 years, a fully loaded cirrus or a 40 year old twin?
You also need to consider your own personal fit and feel. Would I buy a million dollar plane over a 500k plane just because it came with auto-tinting windows? No. But as you go down in price the desirable margins become much better contenders. I dont think theres any question that most people would take a brand new cirrus at 500k over a 250k used twin unless they had a very specific need for a twin.