Evjl's Rain

Level 44
Content Creator
Malware Hunter
firstly, if users only browse the web, gaming or do some office work, WD or MSE can be extremely light for them because they have no web filter
any AV with a web filter is considered to be heavier than WD in this case, obviously
many AVs have their web filters slowing down web browsing noticeably. With some tweaks and workarounds the speed can be vastly better
if users are producers, coders or they do heavy tasks, WD is not for them because it will lengthen the processing time especially while exporting projects

secondly, people consider something lighter than the others when they have not yet tried lighter AVs (with proper tweaks) or they don't have enough patience to wait until cache collection finishes (which can take a few days, up to 1-2 weeks). The cache generation process can take a lot of resources or disk IO => people uninstall the AV in 1-2 days
after this process, the AV will be much lighter
WD almost doesn't have such caching mechanism or there is but insignificant => can be used straight out-of-the-box


Level 40
Bitdefender, if these are your only two options.You might find BD runs well on your system and it's set and forget only. Or, you could use MSE and (even though you said "no tweaks") use Hard_Configurator's Windows 7 profile which is quite simple to implement (two buttons in GUI) and is a combo that definitely won't weigh you down. The choice is yours, my friend! :)