The 45g is comparable to a red switch board, but I would say better, just because of the weight being at the top. 55g is a light version of black switch, and I would say much lighter. The feeling with the weight being that top of the switch compared to a cherry board is a huge difference. It's just a matter of preference. More people will probably favor the 45g, and I would say that is the safer bet for the one you would probably like. Whether the tenkeyless is really worth it or not, is really just a matter of opinion. I prefer the tenkeyless for the additional space (and I never use the numeral pad on keyboards). The price difference is crazy though, I will agree with that.
Once you start getting into Topre though, you can forget about saving money. The keys are very expensive to buy as well $110 a set on EK. But, if you love Topre its all worth it. Again, I would say 45g is the safe bet for most people. I prefer my 55g though over the 45g. But, on occasion love the 45g for certain games, and for typing its a nice/comfortable change. The difference of the two is VERY noticeable, the bounce back on the 55g domes is much stronger as well.