The iPad any's diminutive screen makes it a complete nonstarter and any keyboard that would fit such a screen equally a nonstarter, couple that with lack of cursor control, mouse support, file system support, etc. etc. etc. you are not left with much, and there isn't an app for that.
Surely that is the whole point about how people use devices differently?
The iPad screen is optimised to the size it is for a reason. People who then want to attach a bluetooth keyboard end up with a heavier device with a tiny keyboard, a waste of time. If you changed the size of the iPad then it would lose the effectiveness it has as a tablet somewhat. If you look at pretty well any purpose built tablet, the width is smaller than the Surface 2/3. The Surface 3 in fact is 1/2 inch narrower than the Surface 2 but still wider than most any tablet I know of. The reason is simple optimum tablet size is great for using as a tablet as you can thumb any part of the screen but sucks if you want a decent keyboard attached, the same reason the Surface is wider so that you can. Whether you want an iPad or Android both are still better for tablet use than Windows 8.1. I haven't used Windows 10 yet but I can't imagine it will change things much on that score.
I just use mine for web browsing, the occasional email, video and so on and it is perfect. I spent two months trying to use the Surface 2 to replace an iPad and actually sold the iPad but ended up buying another iPad.
Most people have enough money these days that gadgets are not really that expensive. You compare the cost of these things to say a car where people spend an absolute fortune on just special wheels for their cars and the like. It makes sense therefore just to have different devices perfected for particular uses, so I have tablet (iPad), work laptop (SP3 i7) and desktop (i7-5820 (overclocked to 4GHz) with 16G RAM and 2 SSD and three HDD drives in it). Each does what it is supposed to do. It all gets backed up onto a NAS. I also have an Asus F202E and a 2013 MBA 11 which gets used for work occasionally (but not much since I got the SP3).
My wife just wants an ultraportable device so she uses the Surface 2 (for another few days until the Surface 3 arrives) and something with a decent keyboard on it (hence the iPad is a waste of time). I am interested in the Surface 3 and was considering trying it for a week in place of my SP3 but I probably won't. I had to do without it very recently when I sent it back to Microsoft because of the yellow line down the left of the screen. I ended up using my Asus F202E W8.1 Celeron powered laptop for over a week. It wasn't a great experience, but it did the job, and I was very happy to have the SP3 back. It made me realise that you do take performance for granted once you have it. The screen was also lower resolution and smaller which made more difference than I thought
One thing that does annoy me is the way IE always wants to completely refresh each page you move BACK to unlike iPad Safari. I'm hoping with the new browser on Windows 10 that they've fixed this. I suppose on the Surface 3 you can use chrome which I couldn't do on the Surface 2.
I notice my wife when she does use her Surface 2 on the couch always keeps the keyboard attached. I did find removing the keyboard awkward in the sense it has to be stowed somewhere so my SP3 is almost always used with the keyboard attached. I think my wife is the ideal candidate for the Surface 2/3. Someone who definitely wants a keyboard, isn't too fussy about performance and wants to use office and view documents and the like on a reasonable size screen.
The question of the thread is about having both the Surface 3 and the Surface Pro 3. For me I doubt it as I think the Surface 3 would annoy me due to lack of performance as a work computer while the main reason I'd consider the Surface 3 would be to replace an iPad. I don't think it would do that either. I have been known to change my mind so I am going to have a play with the Surface 3 and will be interested to use Windows 10 as well (but I will wait until it is released, I hate buggy software).