Yeah it could be the marquee itself...is photo-printed glossy.......I'm pretty pleased with the results so far, but its not quite ready for prime-time...I'll snap some pics in a few days when I get it just a little bit prettier 
Sorry for bringing this thread back on topic

.
But the reason you aren't getting the glow you are after is probably the fault of the heavy photo paper. Most of the good stuff is designed to
not let light through it.
But to answer the original question, LED's probably wouldn't do so well due to the narrow view cones associated with the high output variety, but if they were back far enough and you had about 30 of them, you'd probably have the longest lasting marquee lamp available (11 years constant on).
I'd avoid the EL material. It usually has a half-life of about 5000 hours (it never dies completely, just gets ever dimmer). It's very expensive for a large chunk (about $50-75 or more for marquee size) and will require a good sized (also expensive) inverter to power it.
But what might be an interesting alternative are the CCFL lamps. These put off a good amount of light, last a long time (20,000 hours?) and are relatively inexpensive. They also run off 12v and can be wired to your computer's power supply.
*edit*
The CCFL *inverters* run from 12v, not the actual lamps.

These can be purchased with the tube as part of an inexpensive kit.
RandyT