I will try to split into groups to help.
1) Freeview uses standard aerial and only requires a box costing about £20 to convert old TV to be able to receive. It has about three channels not available to freesat users as already said Dave, UK History and Fiver US. but in total far less channels than Freesat.
2) Freesat starts at about £40 and although it loses about three channels to what Freeview has it gains about 40 useful channels. Many news channels plus Films of Men etc.
3) Both Freeview and Freesat also have more expensive boxes with Freesat these jump in price to around the £100 mark and for that extra money you get a 7 day program guide, HD channels, and ability to record programs to a USB stick. Also with some internet connection allowing you to watch BBC iPlayer etc using your internet connection. See
this as an example.
4) Sky this has same program guide as expensive Freesat, It has nearly the same channels. There are one or two that Sky boxes will not get. There is one that broadcasts Elvis 24/7 but unlikely anyone would miss not getting the odd one or two Sky can't get. Plus you also get according to package extra programs like UK Gold, Dave, etc.
5) Many Sky packages also have duel receivers so you can record one and watch another this includes recording two and watching one received earlier. After a power cut it does not lose memory so will still record any programs selected once power returns. It also stores all program info with recordings. Down side you pay each month and even on stand-by it still uses around 20W 24/7 as LNB's are powered at all times.
Since when you up-grade your TV you will get Freeview it seems a lot of money to pay for a box that a few years down the line will end up in the bin. The cheap satellite receivers work well from Lidi,
Screwfix etc. But the electronic program guide on the cheap units is useless. The TV mags seem to miss out loads of channels Films for Men and Films for Men 2 for example. So unless you never watch old films then I would say worth the extra to get 7 day guide. With HD costing so little extra with internet as well I would say if you can afford it the Technisat I have given link to is a good start point. The next is one with Hard Drive and excuse to pun but the Sky's the limit.