Two questions, first one is how much HDD space on average does recording with these take up? I plan on recording commentary over gameplay to put on youtube(just like 8 million others). I read somewhere that it can be quite intensive and my recording HDD has 400 GB max capacity minus the gameplay that's being recorded which can be quite a lot.
Second question, I have a laptop which does not have an XLR port so a direct connect with the at2020 would be a no go but I could get a XLR to USB to go with that. I've read some reviews for XLR to USB converters and it seems that it's a 50/50 works or doesnt. The at2020USB is ~$130 on amazon and non USB is ~$80. Would it be better to get the USB one or the XLR and get a converter?
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Not having done what you are trying to do I would say that the USB mic would make your job easier. Since you are only recording voice you can record your commentary as an 96kps birate mp3 file which would be pretty small. Would not add much to the gameplay recording size and the quality should be adequate.
If you record in a higher bitrate you can allows convert it to a lower bitrate but not vice versa.