2.8 v 4 is quite significant if you shoot indoor a lot. I find it very, very hard to deal with indoor shots cause I have a 55-250, but I'm contented. Haha. Well, you could increase ISO to compensate, but it depends on what body you're using. If you're using a 1000D like me, don't think of touching on 1600 unless really necessary. The picture quality is horribly affected.

Btw, IINM a simple rule for steady shots is 1/focal length. In your case, which would be 1/200 without IS. IS helps tremendously with telephoto shots (we're bound to have jittery hands no matter how professional we are, we're humans after all), and I think you'd manage 1/50 at 200mm just fine. There is no set calculation, just formulas to help you find out the best settings to use.
Both are equally good performers. Since this is so, it's down to other factors. Consider the focal length, the product (it's new, or out for awhile already) and the usability. Different people have different preferences.

Good luck making up your mind, these two are really wonderful cameras.
Mmm, long term planning. I like the way you think.

Well, since you mentioned, here's the breakdown.

1. You might have a chance with Micro Four Thirds system as it looks to have been revived. Is it a fad or a real future builder for cameras we don't know, but I think it'll be a big hit. Your biggest chance would be on this, out of your three choices.

2. Pentax? To be honest, I am not being brand-promoting or anything, but if you ask me, I wouldn't go for Pentax, and stick to the majority. Finding lenses here in Malaysia is so hard, and I'm not sure if their CS in Malaysia is any good (if they do have one).

Depending on where you're from, it'll affect your choice a lot.

3. Full-frame. It seems like quite a big leap for me. Nevertheless, a good idea, but not sure about the lens choices. I think you shouldn't start off with the 50 1.8 first, and get a normal kit lens or something. A normal walkaround for you to familiarize first.

And about your photography style, you'll need a few lenses, just so you know. An ultra wide-angle for scenery and building; portrait, macro and product should be able to be covered by one lens. But an extra macro lens won't hurt. 

I hope I've helped you a bit in making up your mind, mate. (:

Headache to choose between 5D mark I used around RM 4.8k wif battery grib and Sony A850 RM 5.5k new.

How u guys think? >___<