The first advantage you would notice from upgrading your CD player is that you'll discover that the "redbook performance" may be better.
See other thread for definition of redbook. Basically, your Marantz player, while it may have been a good buy for you at the time, is not as good as many CD players made today including those which also play SACD.
The other advantage, of course, is the ability to play SACDs. SACDs use a different method for encoding discs and thus produce fewer/different artifacts than normal CD. While SACD format does not guarantee a good recording, many people have reported less digital artifacting, less glare/digital haze, better texture/air surrounding instruments, more extended highs, more solid bass. These are the same kind of differences you would experience from upgrading CD players at all but SACD goes a step further by correcting some of the problems inherit to the format itself.
Technically SACD's advantages surround the fact that it has a higher sampling rate and a more even distribution of sampling and that it has a larger frequency response. It is theorized that the higher frequencies, while directly inaudible to most humans, have a domino-like effect on the sounds we do hear and impact the subjective quality of the playback.
There are never any guarantees when a new format launches. It is for this reason that I never advocate even though I myself now own a player. My advice is to look at the SACD software available today and see which software you would purchase. If there is music you would buy, consider that when you're deciding between a new player that has SACD versus one that does not. Whether SACD becomes a "dead format" or not, in my opinion, a lot of great recordings are already out and many more will be coming throughout this calendar year.
If this didn't answer your questions, give markl a chance to post in this thread.