The Turbo is litterally twice as big as a PCE just because they thought (probably correctly at the time) that Americans like stuff that is big for no other reason that it being big.
The official NEC reason for making it bigger was to comply with FCC rules of the day regarding RF interference. My mind is still boggled about why this would be an issue to anyone anywhere. Crazy FCC.
The PC Engine is definitely better looking, just as long as you don't hook up anything to it other than a controller and a game card. Want to hook it to a TV? Well then you need a cable awkwardly protruding from the SIDE of the system (of course the Turbo also suffered from this). Want to step up to the sky-high quality of composite video? Slap a
GIANT AV booster on the ass of the machine (do composite video, stereo audio, and a battery save really require that much extra space?) and suddenly it doesn't look so cute any more. The TG-16 booster looks better. Want a CD-ROM drive? That suitcase thing for the PC Engine is freakin' UGLY! And fairly large in every direction, it seems. The TG-16 attached to the CD-ROM drive (which can hold over 4,000 megabits!!!!) looks odd, but it still does look more aesthetically pleasing and, well, less "sloppy". I don't know about it being better looking that the Genesis 1 atop the Sega CD 1, though. That setup looked pretty cool. The Sega CD 2, on the other hand, looked like ass no matter what it was attached to.
I have spoken. And so it is written. errr... yeah.