I currently only have a prizm Calculator but want to buy
a TI calculator, because I think i should have both in programming.
What is the best option?

This is obviously based on opinion. I want to present only fact, and let you choose.

The three "golden" (most used) calculators are the TI-83+ and TI-84+, and TI-84+ CE.

The 83+ is the cheapest of the three, running you only about CAD$100, or <$50 on ebay.
Pros:
- If you write a program for this calculator, it's more likely to run on the 84+ than the other way around
- The button layout is standard and great for playing Fruit Ninja
Cons:
- It's about 2.5 times slower than the 84+
- Waaaay less Archive memory
- No clock
- Less functions
- No MathPrint
- No USB (you can still connect it to other calculators, or the computer if you own a SilverLink)
- It can't run C code

The 84+ is probably the best for your use case, programming. It costs about CAD$130, or <$50 on ebay.
Pros:
- 2.5x faster CPU
- 3x more Archive than the 83+
- Real-Time-Clock
- USB (you can connect accessories, and use any USB Mini cable to connect to your computer)
- MathPrint
- Can run most programs
Cons:
- MathPrint OSes also tend to make the calculator slower and less stable
- It can't run C code
- Button layout is less standard than what the 83+ has

The 84+ CE is the newest of the three, and seems to be the most popular right now. However, TI has recently locked down on ASM code for all models of the CE, causing you to have to use an exploit to run games and programs on newer OSes. It will cost you about CAD$150, or <$50 on ebay.
Pros:
- Color 320x240 display
- Much faster CPU than the other two
- MathPrint
- USB (you can connect accessories, and use any USB Mini cable to connect to your computer)
- Real-Time-Clock
- 3x more Archive than the 84+
- Rechargable battery
- You can develop in C
Cons:
- If you update your calculator past OS 5.5, you need an exploit (currently arTIfiCE) to run any ASM programs or games
- It can't run ASM code from the last two calculators

Of course, there are other calculators as well, but I haven't used any outside of emulators, so I can't give recommendations on those.

Please let me know if I've missed anything.

It always depends on what you're maximizing for. For messing around with programming on the go, I'd definitely say the TI-84 PCE is the best. I carry mine around when I just want to write some fun little programs in BASIC or ICE. For doing math, I usually use my Nspire CX-II CAS, which is a very powerful math machine if you know how to use it, though I know many people on here prefer the HP Prime. A middle ground would be the TI-89T or Voyage 200, an older calculator that has richer programmability than the 84 series (notably being able to make functions and use a CAS) but not as expensive or hard to navigate at the Nspires. If you're looking for what calc to buy next, I'd definitely recommend the TI-84 Plus CE, or the TI-83 Premium CE if you want the rudimentary exact math engine. If you're like me, though, you'll end up with all of them eventually!

Numworks.