In essence, both arguments boil down to this: Should the US adopt interventionist legislation to online content, defending it against international piracy along similar lines to how countries such as China or Iran protect citizens from ideas that their Governments believe are unpalatable?
Taking a purely commercial view it could also be argued that commercial tail is wagging the ethical dog as the laws could in theory force the US government to do the bidding of a few specific industries by taking down offending sites and services that are potentially in breach of both acts.
Superficially both PROTECT IP Act and SOPA appear to be fairly harmless and even beneficial. There are many services such as MegaUpload and RapidShare, which theoretically store millions of copyright infringing files that, if the acts were passed into law, would be forced to go the way of the dodo. According to proponents of both acts, this would allow for legitimate services grow and for content creators earn money that'd be otherwise lost.
Unfortunately the devil is in the detail, and detractors counter argue that the proposed acts are in fact a slippery slope to internet filtering which could potentially do more harm than good.
The bigger problem here however is that there is no way to 100 per cent block all copyright infringing content from being posted, other than shutting the offending service down. Given the tens of millions of files posted on services such as YouTube on a daily basis, there is no way that these services can hope to proactively take more than a mere fraction of any copyright infringing content down. Making matters worse, the legislation also opens up a can of worms by giving copyright holders new legal argument to litigate along the lines of: If the online service knew about one bit of infringing content and removed it, then surely they'd arguably know about other copyright infringing files and remove them too? Clearly there is scope for considerable legal debate, and it is arguable that the only real winners at the end of the day are likely to be lawyers.
Another issue that the proposed legislation hasn't taken into account is the fact that the new laws are also likely to be gamed as it wouldn't take much to fool the content checking algorithms and/or human checkers used by online services.
Add in a few seconds of silence to some uploaded audio, or some public domain video to a movie/TV series and it could easily be slipped past most detection processes. Based on this, it is almost certain that over time, many online services will run foul of these proposed laws.
Adding to the looming car wreck that could happen if these acts were to be passed , there is also significant scope for the legal system to be bogged down by massive amounts of litigation as every audio or video recording ever made is effectively copyrighted.
So If you film your kids, cats, dogs, budgie or goldfish, you own the copyright to that video and under the Protect IP or SOPA acts you'd be able to force online services to take the content down.
Multiply this across the sheer number of people in the US who are potential copyright holders and there is serious scope for a major litigation blow-out and the demise of many online services that make the internet the vibrant and wonderful place it is today.