You'll probably find the SFX cubes to be far too confining in the not-too-distant future. Nearly everyone I've spoken to who's had one for more than a year felt that way. You could get an easy-to-carry Micro ATX tower with a handle on top, they have some that are narrow enough that the CD-ROM has to be laid sideways. That's a lot narrower than a cube, so it won't bang against your leg when you carry it. And it wouldn't be heavier either. Certainly it would be taller, but not as tall as a full tower, as easy to carry as an overnight suitcase.
Look at the situation, a narrow Micro ATX tower will be:
1.) About the same weight as a cube
2.) Thinner than a cube, thus easier to carry by its handle.
3.) Offering more expandability than a cube with 2 more slots available
4.) Providing easy to find and upgrade parts, including the motherboard and power supply.
The major weight factors are the drives, and both the SFF and a Micro ATX thin tower use the same drives.
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