Denon DL304

Pauly,

The stock arm that comes on an Apollo, probably
aluminum tube not magnesium.

I have heard nice things about the Shelters and you
can get them new. Not sure if DL304's are available
anymore!

Have no idea what the issues were with reliability; saw a few people mentioning this on one of the various analogue forums. I was unable to find one to purchase new in any event.
The Shelter vs the DL103R might be akin to a very, very good solid state Class A amp vs a quality SET tubed amplifier. The denon sounds a little "fatter" and the Shelter sounds "crisper". They are both very good; Shelter has a bit more output than the Denon but both are consideredd low output.

Denon is supposedly more difficult to marry with an arm due to its higher VTF requirement and compliance rating. I use a SME 312 which works well with both cartridges and sounds very nice.

I also have a Dynavector DV20XH but it is not as good as either of the above cartridges. I have it mounted on a Yamaha PX3 turntable which is part of the hometheatre.

I recently acquired a Denon DL103m.  If you have a preamp or phono stage that has enough gain for this cartridge it will give cartridges costing 4 figures a run for their money.  I just love it.  Unfortunately, it is long out of production and only available used.  If patient you should be able to find one for $300 to $400.