Windowed is typically about ~5% fps drop, input lag feel may vary a bit on PC setup, for example mine feels virtually the same regardless if I use borderless windowed or fullscreen other than the slightly lower FPS for some people it might be pretty bad.
Alt+Tabbing out of a Borderless windowed might be a bit weird in multi-monitor setups. In my case the game remains in the back ground and it makes it an issue to where I can't see my desktop on my primary monitor until I minimize the window (Windows Key+Down Arrow)
My recommendation, try borderless windowed if the input lag/lower FPS is not noticeable/doesn't bother you and you enjoy fast alt+tabs or have a multi-monitor setup I'd use borderless window.
Some helpful notes if you go borderless windowed.
-If you enjoy a non-native resolution for TF2 set your desktop to that resolution as well so the borderless window is full screen. Your video card control panel should have options for centered timings and if you want use stretched etc etc. SIDE NOTE: make sure your desktop is set to the same refresh rate as what you want to play
-some key commands (Windows Key+Down Arrow to minimize selected window) (windows key + m minimizes all windows) (shift + windows key + m Undo minimize all) (windows key + D Minimizes all open windows and displays the desktop)
-your only choice for changing brightness/gamma in TF2 is your monitor OSD or video card settings. Which is kinda lame :/