Before you buy a generator, it's useful to figure out what you want to do with it. It's also useful to consider buying several generators of different types to handle different tasks.
An arbitrary waveform generator seems like a safe, "limitless" option, able to handle all tasks, but that's not actually the case. If you want a really clean pulse or square wave, with no Gibbs phenomenon ringing and a fast rise time, the arbitrary generator cannot do this. If you want to generate a sine wave with extremely low distortion, the arbitrary wave generator can't do this.
If you're just starting out and have no generator at all, it's not a bad thing to buy either of the two devices you mentioned. But also consider that it's OK to have a handful of other generators for specific purposes. And, sometimes you'll need two generators or more to rig something up. A great complement to an arb. generator would be an analog generator like an HP 3314A, which will give you flat topped square waves and pulses, as well as some easy to set up traditional waveforms. And, maybe a fast pulse generator would be useful, or some sort of an RF generator that can produce quality RF sweeps, maybe as part of an RF analyzer.
To answer your original question re. the Rigol and Siglent, go with the device whose interface makes the most sense to you. But, if you can find older gear, you might be happier with a clean, working analog generator, and you might spend less $$. I picked up an HP 3314A at a ham radio flea market for $225 and it's a great device. Yeah, not the cheapest, but it's a solid piece of gear that does an enormous amount of stuff, and it's pretty easy to use. Sometimes you can find quality gear from Wavetek, General Radio and other companies that can do the basics really well, and are cheap. Then, even if you want to upgrade, these classics are still worth having around, and will get used with some regularity along with the new box.