Mega vs RasPi - complex project and can't decide between the two

Hi all, hoping for some opinions on whether to use a Mega or Raspberry Pi for a current project. I know the differences are discussed extensively elsewhere, but I'm torn still which will be better for the job. I've built out about 30% of the project on each platform so far (I keep going back and forth) but I need to actually commit to one. So any insight you've got is hugely appreciated!

Things needed for the project:

  • flowmeter reading (voltage reading re: analog pins)
  • keypad login system
  • LCD to display readings, directions (16x2 is fine)
  • initialization options directed by keypad+lcd
  • ethernet connection (POST only, no GETs needed)
  • store ~500kb memory when ethernet goes down
  • 24/7 operation
  • insensitivity to temperature (EDIT: will be using a pretty thermal protective enclosure, but still curious is anyone has seen a lot of real world performance difference)
  • EDIT: cost will not end up being a deciding factor

So far my thoughts are: it's quite a bit of functionality and reliability needed out of the Arduino, but RasPi is a bit overkill, much worse I/O and probably will suffer more downtime.

The two big downs about the raspberry Pi in my book is the lack of real time operation and the boot up time. There is also the much higher current consumption.

However for your project it will be much cheaper to use the Pi as it already contains the Ethernet connection. The I/O can easly be solved with port expander chips.

No electronics are insinsitave to temprature and they both use commercial grade components so you will have to provide a protective environment for them both.

I should have clarified on the 'heat resistant' piece, I've made a few edits to my post.

My concerns about the Arduino are more like:

  1. reliability of ethernet shield and library? So far it has had a less reliable network connection than RasPi
  2. memory. If I have to use an SD card, those writes are likely to delay the user experience a bit

Also, I don't really need any threads, so I feel like one of the big advantages of the RasPi is taken away.

Do you think the difference in processing power makes up for RasPi2's lack of rtos?

Also, I don't really need any threads, so I feel like one of the big advantages of the RasPi is taken away.

No I would not say that is an advantage of the Raspberry Pi.

Do you think the difference in processing power makes up for RasPi2's lack of rtos?

No but there seems little you need real time for here.

The only down side to the Pi as I see it is the lack of A/D conversion for the flow meter, you will have to add an external one.

Grumpy_Mike:
No I would not say that is an advantage of the Raspberry Pi.
No but there seems little you need real time for here.

The only down side to the Pi as I see it is the lack of A/D conversion for the flow meter, you will have to add an external one.

conversion to pulse would be easier than to use external ADC, then count frequency.
if a very high resolution is needed, you will need to go with an external ADC in any case as the ADC is 10 bits

I couldn't decide either, so I use a RPi and a Due on the same project. The RPi handles the display, internet and storage. The Due handles I/O.

conversion to pulse would be easier than to use external ADC, then count frequency.

No it would not be. This is because the Raspberry Pi is very poor at doing something like measuring frequency, because of the Linux operating system. This is something that is simple to do on the Arduino.
However the OP said in the initial post:-

  • flowmeter reading (voltage reading re: analog pins)

So there is little point converting this to a frequency only to have it badly measures. A voltage to frequency conversion is of the same level of complexity as an A/D when it comes to implementing them.