Hey Eric:
My gut tells me the 5mbs would be too slow for imagery but you could always try it and then upgrade the pipe if needed.
I'm sure you know that AGS and AGOL aren't mutually exclusive. You can continue to use AGS internally and AGOL for the public facing side and/or a combo of them. Since you have AGS, it means you also have a certain number of named users for AGOL based on your AGS license (unless you have an ELA, then Named users are negotiated differently.) You could serve up your data from AGS, make it available on AGOL and publish a web app there that consumes the data from AGS. You'd use no credits in that case but you would be pushing your data through your ISP pipe. If your AGS is behind firewalls and if you're using SSL, it gets trickier. The easiest way to avoid those problems it is to publish the data up on AGOL.
I think these days most organizations that are using AGOL or Portal are doing a hybrid of AGS, AGOL/Portal. Obviously if using Portal, you have AGS in the mix. And even groups with Portal often use a combo of Portal and AGOL.
To be honest, AGOL makes it so easy to publish data for public consumption and there is a slew of data out there already from the OpenData project.
In our case, as a water utility, we use Portal and AGS (with a Silverlight Mapzilla) internally. We're slowly getting Portal off the ground. We will use AGOL for a few public facing things that some groups are interested in publishing.
The point I'm trying to make is that you can make this stuff all work together and with AGOL, you have a huge breadth of tools at your disposal, a Portal that is administered by others, scales as needed, etc... I imagine you know that there is already imagery as a basemap available in AGOL. If you're like us, we fly our own Ortho-Imagery, in conjunction with other local gov entities. We do this to get a higher resolution than the standard available imagery. But to be honest, if I think about what our folks use imagery for, I imagine the Google resolution is perfectly adequate. Our 3X better resolution is useful for some entities, like Assessors, but I suspect our field guys mostly use it for reference and orientation.
You can always give AGOL a try. Publishing your own imagery to it is not something I've tried because we're talking a lot of data. But you could serve it up from your own server and then publish other layers to AGOL to create the public maps you want. I'd recommend just playing with it to see what it can give you. Use the free imagery basemaps initially and if it looks like something worth pursuing, then dig into it using your own.
Of course, you've already got your maps published and running so why invent the wheel. But you can also use AGOL as the Portal into your maps. You register your maps with Portal as an existing app and they show up in the Gallery. Lots of options. Where I've seen AGOL really successful is with the smaller entities that don't have much of an IT staff. With AGOL, you've got Esri basically being your IT staff for internet apps with the benefit of all the apps they make available to us now. I imagine you know about the Local Government Solutions.
Best of luck!
