I have had both and moved to the 3803 for no good reason other than a testosterone driven need to upgrade .

Without going into great technical detail and probably missing some of the finer points, here is what the 3803 offers in excess of the 1803:
-30 more watts per channel
-Uses the SHARC processor (more robust audio processing)
-7 Channel vs. 6 channel
-If you use the unit for video upconversion (I don't) the 3803 has 100 mhz and the 1803 has 35mhz. Generally, you want at leadt 60 mhz to avoid signal degradation. I never saw much of a difference.
-More robust remote w/learning capability (check this)
-Variable bass crossovers (1803 fixed at 80)
-A few more audio inputs.
-Can act as a pre/pro if you want to add separate amplifiers
-Better audio DACs

There are also a handful of smaller things that I don't understand.

Having said all of the above, if you aren't going to upconvert video, don't have a need for the more esoteric offerings and are just looking for a good value in a well-built, solid and dependable performer with a budget consideration thrown in and you just NEED something to run your setup - the 1803 is an excellent choice. If you just WANT something bigger to avoid that upgrade or "what if" itch later on, then go for the 3803. You won't make a mistake with either.

There is another piece I'll expose you to on this subject. If you haven't considered it, Denon also makes refurbished equipment available through ecost, an authorized Denon etailer. Quite a number of folks on the Forum have gone this route (including myself) and saved some serious cash and moved up a model or two.

Take a look at their website here . Just a quick look shows the 2803 at an excellent price. You get almost 3803 perfromance at about 70% of the 1803 price. The 1804 and 3803 (in silver) are also available.

Just to make things a little confusing, the 1083 (1 left in stock) and 983 are also listed. These are rebadged 3803 and 2803 models respectively. Denon does this for sale via boutique shops and professional installers. They are exactly the same as the 3803 and 2803.

Just a bit of advice - if you decide to go this route, be sure to call ecost to place your order and to assure they have stock to ship.