I don't think you'll ever be truly satisfied with your photos unless you do invest in a digital back. In my opinion, which back you choose is not so important. More in a minute.
The moment you see your first shoot with your digital back you'll be so excited at the quality that you'll forget the expense. If you've managed to beg, borrow or steal enough to own one then life goes on but with the pleasure of photographing with a digital back.
I was shooting MF, mainly with a Pentax 67 system before digital came along. I bought myself a 10D when it came out thinking that I would do not so important (magazine!?) work with that and good stuff on film. But, we all know the story... the advantages of being the master of my own retouching, colour, exposure tweaking, not to mention the extra cash that stayed in my pocket from not having to buy, process film, meant that I stopped using my medium format camera.
Slowly but surely I stopped feeling like a photographer. I didn't get anywhere near the pleasure of shooting with a DSLR that I used to get from shooting with my pentax 67. I was spending a lot of time in photoshop (which was an important phase), and the pleasure I was getting as a photographer was really infront of the computer, but not so much while I was shooting.
Two and a half years ago I decided that the only way forward, for me, was to take 30 000 I did not have and invest in a phase one P45, which I put on a contax 645. I remember that first shoot and looking at the images in capture one (P25 back at the time, before they gave me the P45), and then once processed and being blown away by the quality. I also loved the heavy feel of the shutter and the big mirror slapping inside the contax. More to the point though, the images were far beyond anything I was taking with various dslr cameras I had tried to use.
If a pro shot on LF before digital came along then I imagine they have moved onto shooting on a large format camera with a digital back. I shot everything on MF and can't imagine shooting on anything else. I thought of trying to shoot with my P45 on a LF camera, but, because I hardly ever shot any LF before digital then it is still alien to me.
If you have never shot on LF then I would recommend first trying out a MF system with a digital back with a 35mm lens and use photoshop for the perspective. If in time you start shooting only architecture then you'll probably want to put your back on a LF camera. LF lenses are better than MF lenses are better than dslr lenses - I don't think too many people should argue with that. But, and I think it's a big but, LF photography takes time to set up a shot, and if you're taking the kind of interiors you show on your web then looking through your viewfinder, taking a couple of shots, moving down a bit, across a bit, taking a few more shots, then moving a bit more...is a lot easier with a medium format camera.
From the work you have on your website I think you don't necessarily have to shift and tilt to get what you're after. I think your images are good in regards to composition and architectural feel. If you shot them on a digital back then you would suddenly get detail in your highlights and lowlights, and there would be more tones to your colours. You won't have to do anything different to what you are doing now, but you'll feel that your photographs are much more to your liking.
Stacy, please feel free to take a look at my website. www.stefansiba.com
. It's a prototype which will be changing in the next month or so, but the portfolio section and the client section have an example of architecture work taken with my contax 645 and 35mm lens with the P45 phase one back. I've been rambling a bit so will wait for any reaction to carry on. And, without the couple of glasses of wine. Good luck with your decision making.
My answer to your initial question is that if you invest in a MF digital back then you won't be disappointed, and you will most probably start taking the kind of photos you want to be taking. IMHO, if you try to take architecture images with a dslr you may never be satisfied. Tilting and shifting, whether done in camera, or in photoshop, is not as important as the feel of your images. The perspective you will sort out one way or another.
Just a quick further point while I have it on the tip of my brain - when I started using my P45 I kept on wanting to go to photoshop to retouch and finish my images as I'd been used to doing. But, once out of capture one there was often very little to do.