The Rigol is brand new, the Tek is used, but both at the same price. Which one would you go for?
That depends on what I'd wnat to do, really. Because this isn't a decision between equivalent scopes.
The Tek TDS540D was a scope sold at the end of the '90s and therefore is pretty old. Some variants also suffered from bad caps which tend to leak and in the process often also damaged boards beyond economical repair (although I'm not sure if the D variant was affected). You get 500Mhz BW and 2GSa/s (1GSa/s in four channel mode) as well as up to 8Mpts. Due to the slow architecture high waveform rates are only achieved in a special mode where the scope reduces the amount of data it processes and in which no measurements can be done. In normal mode the waveform rates are quite low, though, and intensity grading is limited to 16 levels only. The good thing is that the TDS540D lacks the NuVision color display (which is pretty much a monochrome CRT with an LCD shutter in front of it to create the impression of color) of the bigger models, as many of the shutters tend to have deteriorated or failed over time. If the TDS540D works it's an OK scope but it can be a bitch to repair.
The Rigol DS1104z is a low-cost bottom-of-the-barrel scope with 100MHz BW (and if hacked this only goes up to some 150MHz), which isn't really comparable to the 500MHz Tek. It's smaller than the Tek, draws less power, and comes with a modern widescreen color TFT. The Rigol also has more sample memory as the Tek, and can do some simple serial decoding. And like any decent scope it doesn't rely on a special mode for high update rates but achieves them in normal mode. When it came to market the DS1000z Series was full of bugs but by now most of them seem to have been fixed. And if it fails then you're covered with 3 year warranty.
FFT is pretty basic on both scopes, and neither come with any decent search or analysis tools for data in sample memory.
If the choice has to be between those two scopes only then I'd say the Rigol is the better offering unless you really need the higher BW the Tek offers (in which case I'd have a look at other possible alternatives).
These are a great scope even compared to todays scopes.
No, not really. I understand that for someone coming from a Hantek this is probably a huge upgrade, but reality is that these old scopes can't hold a candle against a decent newer scope.
If you want brute performance on a budget these scopes will still out perform your rigols and siglents by a long mile.
Only in BW. Rigols and Siglents have a lot of issues (mostly bugs) but in general their scope platforms are faster than those old Teks.