Durallymax has provided the most complete solution thus far, and I think that would be adequate for what you're trying to do. However, I'd like to add a couple points for consideration.

The Click PLC definitely going to be more rugged than an Arduino, but industrial/commercial standards are still nowhere close to Automotive/Military levels. I'm assuming you'd be installing your equipment on the baler itself, where it will be exposed to dust, humidity, a lot of vibration and harshness, and potentially moisture, to name a few. I don't see a harsh EMC (electrical noise and interference) environment on a small square baler though. A stock Arduino is not hardened at all for these conditions, where the Click PLC would be better. Mounting either an Arduino or PLC inside a good enclosure or cabinet would help, but vibration and temperature would still be issues.

A simple search turns up this company that claims to make "ruggedized" Arduinos: https://www.rugged-circuits.com/products. After examining some pics of their Ruggeduino, I observed a lack of copper balancing and amateur component pad design. I can't speak to the components, as I didn't see their BOM. However, I'm sure it would be better than a stock Arduino and would likely be just fine in your application. 

Here's an Arduino based PLC: https://www.controllino.biz/. This device can apparently be programmed using either a PLC language or using C/C++ as any Arduino. However, this is not really different than the Click PLC, it's better suited for industrial applications rather than mobile equipment.

There are also many other development boards out there other than Arduino that can be programmed in C/C++ and are available in varying levels of ruggedness and features. Some searching should reveal more options.

Some of this question comes down to how you want to handle programming. If you prefer to use a PLC language such as ladder, Functional Block Diagram, Statement List, structured text, etc, then you will likely need to go with a PLC solution. However, here is a solution for programming any Arduino in ladder: http://soapboxautomation.com/support-2/soapbox-snap-tutorial/soapbox-snap-arduino-tutorial/, I have no experience with this, YMMV. 

Personally, I'd go with a "rugged" Arduino, make or buy a good enclosure for it, then program it in C++. As someone mentioned, the Arduino could store the data in a delimited text file on an SD card, which you can then later manipulate with Excel and create charts and visulizations etc. If you go another step futher, add a Bluetooth shield and write a simple phone app to see your bale information in real time. I'd still use the SD card for logging though.

If you choose to go with Arduino and need to get up to speed on programming, I'd recommend this tutorial series: https://www.youtube.com/watch?v=olpIWDYKW00. This guy is teaching from an automation and machine control perspective.

As for sensing position, using an IR proximity sensor or hall effect sensor could work. If you're sensing an action like a knotter cycling, then a third option might be to mount a microswitch if you can find a good place to install it.

I can't find the article anymore, but I was reading about a Fendt system (concept?) where they record info such as moisture, time it was made, flakes per bale etc and record it to a chip embedded in the twine on each bale. This project just reminded me of it.

These are just my opinions, I don't think there is really a right or wrong way to go about it, but I do agree with others that the Raspberry Pi is definitely the wrong tool for this job. My day job is in power electronics engineering for automotive applications, hence my emphasis on designing for the environment. We could go much further down this path, but for now I think you'll want focus on getting a working solution, then optimize further as needed.