Does the difference in price justify IC?
It really depends. Right now, EIC makes sense because you're guaranteed air conditioning and a lack of overcrowding, whereas TLK will always have that "uh oh, what if it's rammed?" factor combined with older rolling stock. The journey is usually a bit more comfortable as well...BUT - sometimes they use EIC stock on TLK services too.
It's worth noting that you can pick up EIC tickets for peanuts if you're willing to plan ahead/take a chance.
Journey times are almost the same really, it's all about comfort. I'd say - at non peak travel times outside of summer, there's no difference.
The other thing is that EIC trains tend to be...how to say - free from students and the lower part of society. And this is worth paying for ;)
(really, it just depends on the route, time of day and time of year)
InterRegio trains can also be worth a shout on certain routes.