I had this same dilemma when I was trying to decide on a tour to Europe. I have friends who’d been on both Contiki and Topdeck and everyone came back with massive smiles on their faces no matter what tour company they were with So I knew it wasn’t going to be easy!
So I ended up doing the age old system of picking key criteria and then weighting their importance etc etc. I looked at cost, number of nights, number of countries visited, which key places I wanted, average ages and inherent reputations…
I wanted the cost as low as possible but definitely under $4000. I wanted over 24 nights but under 30 and I wanted to fit in as many countries as possible. One of the biggest things that tipped me towards Topdeck was the number of countries they fit in (26 days Topdeck fits 13 countries whilst only 11 were on contiki for their 25 day). I also loved the fact that (on my tour anyways) they go to Eastern Europe eg. Croatia, Albania and all the " less tourist saturated" locations. :-/
I’m also 26 and when I spoke to both the Contiki and topdeck representatives at a travel expo, the Contiki guy told me that the majority of travellors are under 23 AND single…I think he picked me for younger than I was…well needless to say that gave them at least 1 minus right there. PLUS my friend had told me that quite a few people were regularly hungover on the Contiki tour (she went on the contiki tour)… well I like to party just as much as the next person but I certainly didn’t want to party until I was drunk - esp since I wanted to spend my money on more important things like shopping lol. But I guess you can get those kinds of people on any tour…
In the end, the deal was sealed when Topdeck offered a massive saving at this travel expo! It was too good to be true, I just couldn’t pass it up and I am CERTAIN that I made the best decision ever
Good luck with your decisions! B-)