I'm not sure why it is I must make a choice to use one or the other?
Well of course, you don't have to it's just that it makes no sense to use both. It's a bit like trying to write your novel in two different word processors - it's possible, but makes more work, is unecessarily confusing and makes for needless complexity, but hey, if that's what you're into...
I'm not using iphoto at all but Aperture instead (much the same in the sense they are both photo viewers...
Neither are "photo viewers" and and that might be the start of your problem. They are both databases. When it comes to editing the principle deficit is compositing... this sort of thing:
Lets say I need to create a collage of four images, with a logo and a bit of text.
That's exactly what you would use an external editor in either app for.
I really do not understand why apple do not allow you to see into your iphoto library? Take finder for example, say I want to upload an image to the web. Usually you click upload, the finder window pops up, you look in the left hand column for your photos, click on the 'pictures' tab then onto iphoto library.
That's because you don't understand what iPhoto and Aperture are... You think they are photo viewers like Bridge, but they are not. They are Photo Managers built around a non-destructive workflow. As with any database to get data out you need to export it.
Bridge is a photo viewer, Photoshop is an editor. Not only different apps but entirely different types of apps. The nearest thing in the Adobe ecosystem is Lightroom. A'l of these are database, and like any database, you need to export the data.
These apps replace the Finder for anything you want to do with your Photos. Remember the Finder is a file manager. These apps are photo managers. It's a key difference:
Remember that file is just a container - a box - for your Photograph. Think of it this way: In my iTunes Library I have a file called 'Let_it_Be_The_Beatles.mp3'. So what is that, exactly? It's not the song. The Beatles never wrote an mp3. They wrote a tune and lyrics. They recorded it and a copy of that recording is stored in the mp3 file. So the file is just a container for the recording. That container is designed in a specific way attuned to the characteristics and requirements of the data. Hence, mp3.
Similarly, that Jpeg is not your photo, it's a container designed to hold that kind of data. iPhoto is all about the data and not about the container. So, regardless of where you choose to store the file, iPhoto will manage the photo, edit the photo, add metadata to the Photo but never touch the file. If you choose to export - unless you specifically choose to export the original - iPhoto will export the Photo into a new container - a new file containing the photo.
As for uploading, sharing to other apps and so on, there are hooks built into every single Open or Upload dialogue in the OS. In the case of Uploading to a website the first one of these is what you want:
https://discussions.apple.com/docs/DOC-4491
Your entire library, searchable, with the actual dialogue.
So, what you're missing, at heart, is that these apps are not "Apple's version of Photoshop" or whatever, they're a different kind of app, with different aims and uses.