Both offer really good services, but do not fall for Openweb's "Specials".
From my experience with Openweb, their "Standard" performed really well, with the exception of fairly slow torrent's during peak hours (60-80kBps) - Other than that, really good and close to full speed for browsing, streaming and HTTP DLs. Of course full speeds with all protocols after hours - +100kBps on torrents.
However Openweb's specials are disastrous - I used their "Downloader's Bling" special which is based on their Gold profile, and have had endless problems. It is pretty much useless during the day (Peak time), usually achieving 20-40kBps HTTP downloads and 0-10kBps on torrents. Full speeds after hours though.
My experience with Afrihost was pleasant - achieved full HTTP speeds 24/7, low gaming ping and pleasant streaming. Torrents usually reached ~70-90kBps all the time (Not after hours unshaped like Openweb) - Other than that, really good package.
I guess if you queue your downloads to run overnight (and want full line speeds) then Openweb is for you. If you do not like to be told when to download (not unshaped after hours) and download on the go, then go with Afrihost.