If you think you want the extra gear (10 vs. 9), it will be difficult to upgrade your drivetrain for 100 Euro or less - certainly will cost you (much) more with new Shimano parts at retail. You'll need a 10 speed right shifter, a 10 speed rear cassette, chain, and installation to make the conversion.
Make a difference? Depends on the terrain you ride, but as a general statement - yes, worth it.
Will give you an extra gear on each chainring (big /small) to use without cross chaining. Also can run a tighter ratio of gears (depending on cassette), which can be more efficient on flat/rolling terrain. For hills, the extra position makes a wider gear ratio possible (again depending on the cassette chosen).
If you can avoid the temptation to covet Shimano 105, then the upgrade to Tiagra is worth the [COLOR= rgb(16, 14, 68)]€[/COLOR]100. However, if you will eventually want 105, Ultegra, or DA, then save your money to put toward the final groupset you want. There is a frameset change (hydro-formed aluminum) in the pre-configured Scott Speedster line that jumps the price for the 10 and 20 series with 105/Ultegra (and other component upgrades) to much more than [COLOR= rgb(16, 14, 68)]€[/COLOR]100.