If your using heavy mono then I don't think the SL20 would have enough capacity.
I use the 535/SL30/7500 size for heavy ground with 30-40lb mono - though casting isn't a major factor as fish are usually within 80yrds. For me, the Daiwa and ABU 7500 are smoother to cast than the Penn - though the Penn is built like a tank and more robust.
If distance is a concern then the 535Mag might be worth a look as it can be fine tuned easily for headwinds/bait sizes.
The other reel I'm quite impressed with is the Daiwa Sealine-X 40HV - holds 300yrds of 30lb mono and casts well - though the retrieve ratio is lower than the others at 4.9:1 .
Penn 535 mag is good for what you describe, but it needs to be matched with 28/30lb line (or you will have a nightmare with it. It is too big for lighter lines. The same applies for the Diawa SL30SH or equivalent reels.
Big spool equals big lines usually.
As spellbound says, those that cast with the big reels usually only cast short distances.
If you must have a big reel with big distance, you must match it with big line. The big line will however cut down distance - but thats life, and thats why these reels have big spools.