I've used both services.  Tix4Tonite is not a half priced place, because they put on a service charge.  And they tend to have mostly the lower tiers of shows on sale, which are grossly overpriced to begin with.  If you decide to use them, be prepared to wait in line.  If you have a car with you, you can park by the one just south of the Riviera.  
A better deal would be Gold Star Events online, where you can purchase in advance, so you know what is available.  Their deals aren't half price all the time, but they have a wider range of shows.  I've used them, too, and they send you a confirmation to print out.
I've used Tix4Dinner, and it *is* a deal.  You can purchase these up to a few days in advance.  The only drawback is that you have to make a reservation at the time that you purchase the voucher.  So you have to know when you will have dinner today, tomorrow, or the next day.  You purchase only a voucher from them for a service fee (don't remember how much, but less than $15., for sure) and pay the discounted price with the voucher at the restaurant.
Hope this helps.  Its been awhile since I've used them, but I'd do it again for dinner.
Fern