Different Tour Companies

hi all,

i'm planning a european trip for early-mid next year and am currently doing some research to get a head start!
i only ever considered doing a contiki tour but while doing my research have come across some other tour companies such as top deck and budget expeditions which seem to be a bit cheaper than contiki (especially budget expeditions)....but i'm wondering if there is a reason they are cheap!
i know i will be safe with contiki as they are a reputable company, however i would like to spend as little money as possible!
Advice and opinions from either experieced travellers or people in my situation would be greatly appreciated!

Thanks
Andrea