Welcome to Head-Fi, peaches! With a $300 budget, that would just about cover the cost of a Senn HD-600 from Meier Audio and a JMT built cmoy amp. If you contact JMT (a moderator and amp builder extraordinaire) he can twek the amp to drive Senns. The cost of the ER4 is $269 whoich will put you over budget, if you go for an amp. Or you could get the ER-4P which can be driven quite easily by your portable, and get an amp later. Of course, the sound quality if plugged directly into your portable, would then be influenced a lot by the electronics of your CDP.
As for the sound differences of the HD-600 and Etymotic ER4, there is a huge gap there. They are very different sounding and give a totally different listening experience. The Senns are ultra comfy for most people, and they have a smooth laid-back sound, with very good bass response. They are also quite large, and are not really for use as a portable, although some here use them as such. They Etys, for some people, can be uncomfortable because of the in-ear approach. Others, including myself, do not find this to be a problem. They are a very revealing headphone that allows you to hear everything that is right, or wrong, with the rest of your system. I love them with high quality recordings. Poor ly recorded music can sound downright horrible!
Which you settle on, HD600s or Etys, really depends on which sonic signature you prefer, and also if you want circumaural or in-the-ear. Auditioning would be a good idea. You might want to take advantage of Headroom's 30 day return policy to see which one you like. Hope this helps.
BTW, I'm moving this to the appropriate forum......Headphones.