From the PSR-E453 to the PSR-S770 is quite a wide range, going from a street price of about $280 US to about $1200 US. Of course, there are going to be some significant differences between the two keyboards, but I can share my experiences.
While I have not (yet) steadily played professionally, I have played keyboards for 39 years. I learned keyboard on a smaller Wurlitzer organ, and then a Yamaha D80 organ. I learned electronic music in a high school class that had Arp synthesizers, as well as a 4-track reel-to-reel recorder (this was in 1979 and 1980). I then owned a Yamaha DX-7 synthesizer for several years, and then replaced it with a Yamaha PSR-500, then 510, and then 520 keyboards. I have also owned a few Casios. And now, my main keyboard is a Yamaha PSR-E433, which of course is one of the predecessors to the PSR-E453.
I mention this to show that I am no beginner, and I am familiar with a variety of keyboards and types of synthesis (analog, FM, and wave memory). With all of this, I am always impressed with the sound and features of my PSR-E433 whenever I play it, even though it only cost about $250 when I got it, and I do not feel like I have outgrown it. The E453 takes the E433 and adds more polyphony, tunable scales, additional sounds and styles, DSP effects, and built-in audio-through-USB, among other features.
My feeling is that, when you go to the higher priced keyboards, you will of course get more sounds and styles, and better quality sounds and styles, more automatic features, as well as more options for the sounds and better onboard recording. But for the "meat and potatoes" of it all, the E453 is going to have some very good sounds, which can be made even better with tweaking, as you can adjust the filter, envelope, chorus, reverb and other parameters of the sounds. I believe it is the background styles that become significantly better with the higher priced keyboards, as many of the built-in styles on the E4-series keyboards do sound a little cheap and tinny. I rarely use styles, except for the drum track, and therefore, that is not an important issue to me.
As for recording, yes, the built-in sequencer/recorder on the E4-series is quite limited, but useable. I use it to get the ball rolling on recording a song, laying down tracks for the main melody, chords, bass line, and drums, as well as some effects. It's capacity is about 19000 notes, which I find is plenty. And then, I send the audio to a DAW on my computer (I use Audacity, a free program), which is required anyway if you're going to record your music in a format that can be listened to by other people (or even just yourself) away from the keyboard. Once the recording is on my computer, I then use Audacity to add other tracks, if needed, and at that point, many of the polyphony and recording limitations of the keyboard become irrelevant, as the recording is now being done on the computer.
The PSR-EW400, mentioned by Michael above, is also worth considering, providing 76 keys, an improved piano sound, additional power for the built-in speakers, and dedicated left and right audio outputs.
I might add that, while there is talk of a possible PSR-S770 replacement in the works, I also expect the PSR-E453 replacement, likely called the PSR-E463, this coming spring, based on the schedule that Yamaha has introduced these models in the past, and this could possibly also result in a successor to the PSR-EW400, as well. However, this is pure speculation on my part, and if there is a PSR-E463 coming up, I have no idea what features will be on it.
I hope this information is helpful.