I have a Tascam 414 MKII, similar to the 424, and have made some excellent recordings on it through the years. Recently, I moved to digital, though--Yamaha aw16G--and am amazed at the increase in quality and ease of recording. There was a several-month learning curve, however, which no one should deny although I'd be happy to go through it again, given the stuff I'm now able to produce at home.
The obvious question is this: you can buy a 4-track digital (Korg, Tascam, Zoom, Fostex) for nearly the same price as a new 424.
I hope you'll try them both in a store before plunking down change--you just might decide to go digital once you become a bit more familiar with these relatively simple entries into the digital world. If not, I'm sure either of the cassettes you mention will
serve you well--as long as you realize that the sound quality will inlude some high-end hiss, especially if you bounce tracks or play the tape too much. They are good learning tools, though.
Good luck!
J.
I really like both my Tascams, 424, 688's.