I cannot afford the tower. But for upgrading memory and adding additional HD (external) which is best?

Upgrading memory on either the Mini or the iMac is simple, so doing so is really not a problem.

As far as an external hard drive is concerned, it's really a "plug and play" issue. Again, doing this is pretty easy on both machines.

Although the Mac Mini is the "little brother" of the Mac desktop line, they definitely didn't skimp a whole lot of functionality and expandability. They have plenty of ports, FireWire, Thunderbolt, HDMI and a card slot.

And as far as adding memory, you can upgrade a Mini yourself to 4gb, or buy one pre-installed with 8gb. So you have plenty of options there, too.

So in a nutshell, they are both pretty upgradable and pretty flexible as far as external hard drives go.

Hope this helps.

The Mac is maxed out
.
I got an IMac and the only thing I can add on to it is memory
.
I got a external 1 trig drive
.
That is it
.
Oh you can go the new keyboard or mouse
.
But the mac is maxed out when it is purchased and the only thing that can be added on is a bigger internal hard drive
.
There is no video or sound card that can be purchased as is in the PC computers that run Windows
.
That is one of the biggest problem a PC has is all the different hardware are that can be in them when they are got
.

Memory upgrades are extremely easy to perform in both computers. It's probably easier in the Mac Mini, all you have to do is screw off the bottom and the DIMM slots are right there.

In the iMac, there is a grill on the bottom of the display that comes off, you just slide your DIMM's up there.

Of course, when you order the machine, Apple can go ahead and install it for you.