Wait 550$ is for 50MHz. The 70MHz is a whopping 890$.
For that money you can get a DS1074Z-S, which has 4 ch, also 1GSa/s (I bet both scopes have shared ADC's), roughly 10000x (how many orders....) the amount of memory, 30k waveforms/second + intensity graded display, serial decoding, and a 2 channel 25MHz AWG.
Tektronix, wow.. just wow.. They must have had the laziest product development meetings in the "entry level scope"-project. "Just sell the same shit again with a new UI".
Why memory depth matters IMHO... Single shot mode; capture a burst of data and view it to your hearts content.
You can capture the whole picture at once, and zoom into every detail you need. Imagine a burst packet that's too long to capture at the smallest detail you require. Then you have to pan to approximately where you want to look, single shot that area, analyze it, etc. That only works if the trigger is bang on and the signal timing stays identical, otherwise your signal jitters all over the place.
It makes life so much easier.