I like both of these detectors very much first of all. If you are not detecting in thick shallow iron trash, they will both do fine otherwise. The Simplex and Vanquish are both excellent coin and jewelry detectors and can do well on low, mid and high conductors.
I have owned the Makro Racer 2 and the Nokta Makro Multi Kruzer in addition to the Simplex. Personally, I preferred the Simplex. It has more modern features like fully waterproof, wireless headphones and internal rechargeable batteries when compared to the Racer 2 which otherwise is a great detector.....It has better target ID, stability and equal depth to the Makro Multi Kruzer. It has plenty of detecting modes and some of them have very fast recovery speed and good target separation. My only complaints with the Simplex are: it is not nearly as simple to operate as its name implies, it is very nose heavy with the stock coil, the buttons are hard to push, and the lower stem is way to flexible. Otherwise, it is worth $500 as a detector at least.
The Vanquish 440 or 540 are more comfortable to swing than the Simplex. I am including the 440 because it is such a great deal. The Vanquish from my experience (and I have owned and tested all of them, still own the 440 and use an Equinox 800 almost everyday) is the most target ID accurate entry level detector on the market and that accuracy goes all the way to the edge of detection even on low and mid conductors. The 540 has a great set of features like Bluetooth wireless audio, very easy to learn and operate, easy to use buttons with no hidden duplicate functions, easy to set notches, very tonal audio similar to the Equinox and many Whites detectors and a really easy to read, uncluttered display with a nice red backlight. My only complaint with the Vanquish series is that the user no control over their ground balance settings. Since they are simultaneous multi frequency detectors, they are constantly ground balancing in the background. That is great if you are at the beach or in moderate to mild soil conditions. Where I detect in high mineralization, the built in continuous ground balancing cannot handle the mineralization. I have to discriminate out the 3 lowest numbers on the target ID scale to knock out the ground noise. If you don't have hot ground they work just fine. So in my opinion, Minelab blew it and should of included at least a simple ground grab feature.
If I was going to buy one or the other I would get the Vanquish 540. If I wanted both, I would get the Simplex WHP and a Vanquish 440. Even though the 440 does not have Bluetooth wireless, for the price it is just amazing. It detects as well as the 540 but with 3 tones instead of 5 and no wireless capability. There are a few other differences too but nothing major.
The Simplex and the Vanquish will do well at any salt water beach in both wet and dry sand. The Simplex just got a large selection of coils recently too. The Simplex with its 11" coil and Vanquish (with 12x9" coil) will easily hit 10 to 12" coin sized targets in moderate to mild dirt.
The Simplex might give a numerical target ID on a target that deep. It will definitely give a tone. The Vanquish will very likely ID those deep targets very well.