All brands have their pluses and minuses. The major brands all offer a good variety of products. I think you first have to decide what features you need then want. If you want to hear the newest audio codecs but your blu-ray dvd player does not decode them, then a first priority is the receiver's ability to decode the codecs. Next may perhaps be a second zone, bi-amping, etc.
No piece of electronics is future proof but I do not think we will see a leap forward in codecs for a while as we have seen in the introduction of the latest Dolby and DTS codecs so a new receiver "should" be somewhat future proof at least for the next few years. What follows in advancements may not be something you need or are interested in so don't worry about them...whatever they may be.
People are nearly as dedicated to receivers as most of us are about speakers. We generally all have our favorites. Yamaha, Denon, Pioneer, Onkyo are the normal suspects when someone asks about receivers. Marantz, HK, etc. are next not because of quality but mass offerings. The first 4 are generally everywhere while the last 2 and others are not so much.
I have not listened specifically to the Marantz 5003 but I have listened, recently, to the 6003 and am quite impressed. The differences are very few and I would not hesitate, in fact am saving my money for the 5003 right now, to buy a 5003. I am also an owner of the Yamaha 659...a bit older model but regarded as one of their better receivers...and love it. I do find the Marantz tone qualities to be a bit...not a trained ear by any means...smoother than my Yamaha, which is consistent with most peoples thoughts.
I suggest you take some favorite music and go listen to the receivers then let you ears and budget decide. You should also read the brand/model specific threads...I find the forum members thoughts/reviews/etc. to be generally much more valuable than a review especially because of time of use...not a few hours in a test area but in an actual home and used by the owner.
Good luck and keep us informed.
