It seems that MX-600 with IRclone approaches the price of an MX-800 that computer connectivity already. However, I would need to buy a serial->USB converter since I don't have a computer with serial ports anymore.

What is a smarter move here? I only have these devices to control:

Pioneer Elite 45tx receiver
Sony LCD RPTV
Panasonic S35 receiver
ReplayTV 5040
Motorola DCT-5100 cable box

So a total of 5 devices. I don't expect to get anmore devices in the near future.

Thoughts of which to buy?

It is really up to you. IRclone adds a lot of value to the 600, but it will never be an 800. The 800 can have macros on every button, it can handle more of everything (more displays/device, more devices...)
also in RF the 600 is fixed to IR+RF, while the 800 can not only do one without the other, but you can even tell it from what address to send the IR on the Base.

Like you said, you only have 5 devices and the 600 can handle twice as many. But the thing is that nothing forces a physical device to be a device on the remote and vice versa. For instance, you could make a TV device with all the commands for you and a second TV that only has the most basic commands for others. You might decide to go more activity base and have a "device called" DVDRC and DVDTV where in device one the volume buttons control the receiver for full viewing and in the second one it uses the TV for after hours when you don't want to wake people up