It is not clear what you mean by Piper 180... You might mean an Arrow 180 or Comanche 180, but for the purposes of this post, I am going to assume you mean the Cherokee 180.
But 1st... the Beech. I got my license at the University of Illinois in the early 90s, and at the time, their primary flight training planes were Beech Sundowners (180hp fixed gear, 4 place Beech) and the 180hp converted Beech Sports. The Sierra is a retract version of the same plane, but with an 200hp engine. Interestingly, even though they had a fleet of about 15 Sundowners, and used Duchesses for multi-training... they did not have any Sierras. Instead, the complex plane was a Piper Arrow.
Sundowners have a large cabin with a lot of headroom (you sit very tall/vertical). The downside of that is frontal area and the resultant drag. According to the Sundowner owners manual, WOT on a Sundowner @ 8000ft gave about 124KTAS @ 11gph. I don't ever remember them going that fast. 115-120KTAS is more realistic. I think will translate to the Sierra as well. Even with the retract wheels and 20 extra HP, I suspect a Sierra will not hit 140KTAS.
The Sierra and Sundowners do have the 2 doors, which is nice especially if frequently flying with passengers in the plane.
I currently own a '71 Piper Cherokee 180F that has a number of speed mods on it. Compared to the Sundowner, it climbs better, lands shorter, and can cruise @ 130ktas once I get up to 8k. The cabin is a bit shorter (although I am 6'3" with a long torso, and I fit fine). The Cherokee 180s got a ~5" backseat legroom extension in 1973. That's almost a must if you are going to carry anything besides a kid back there. Having said that, I flew a ~20 hr X-C with 2 other guys to SNF last year. It was not comfortable, but it can be done.
The Cherokee is basically still in production via the Piper Archer family, so many parts are still easily available. In addition, they are one of the most common planes, so any mechanic should be able to work on them. The Sundowner and even more-so the Sierra are uncommon. At this age (many are approaching 30 yrs old) I wonder if parts are an issue.
From a handling standpoint, The Sundowners did have a
small tendency to porpoise on landing due to setting the nose gear down too hard. The rule at the University was that 1 bounce was ok, but to go around if you bounce twice. It would be interesting to know if the Sierras have the same behavior... and if it is a concern given the retract nosegear.
Anyway, I never thought the plane handled badly, and frankly, in comparing 4-seat 180hp planes... they are all about the same. ~1000ft takeoff and landing rolls and 800fpm initial climb.
When I bought my plane back in 2000, I briefly thought about a Sierra and ultimately decided on the PA28-180. Given the choice again, I would do the same.
I am looking to move up in speed and capacity from my 140.
Money is definitely an issue.
Two planes have my attention, the 180 and a Beech Sierra.
I am not familiar with the Beech but I like the 2 doors and the roomy look. I have not seen one in person but the pictures look good.
What are some comparisons and opinions on the two?