For my generation of strength piker. (Which was amazing) I went with a higher strength build to get a ton of killshots, which in turn made me a bit of a target. So carrying that sidearm is a REALLY good idea for when some mook closes in, bash his brains out and go back to ramming the stick of death into them.
Since you're gonna go heavy armor, I'll modify my build around that for GRAET SURIVABIRITY!
Strength: 24
Agility: 15
Hit points: 75
Skills to attributes: 8
Ironflesh: 8
Power Strike: 8
Shield: 0
Athletics: 5
Riding: 0
Horse Archery: 0
Power Draw: 0
Power Throw: 0
Weapon Master: 2
A note on WPF, you can put the remaining points all into polearm or the sidearm of your choice.
One Handed: 1
Two Handed: 1
Polearm: 110
Archery: 1
Crossbow: 1
Throwing: 1
Now, onto sidearms. For 1 slot pike-companions, there's actually a lot to choose from. You aren't tied down to using a one-hander. Since you're strength oriented you can make use of a wider range of weapons and still have effective killing power.
Polearms: Quarterstaff (http://c-rpg.net/index.php?page=equipshop&cat=polearm#!?page=itemdetail&id=111), Shortened spear (http://c-rpg.net/index.php?page=equipshop&cat=polearm#!?page=itemdetail&id=92), Military Fork (http://c-rpg.net/index.php?page=equipshop&cat=polearm#!?page=itemdetail&id=99)
Two-handed: Mace (http://c-rpg.net/index.php?page=equipshop&cat=twohanded#!?page=itemdetail&id=189), Military Sickle (http://c-rpg.net/index.php?page=equipshop&cat=onehanded#!?page=itemdetail&id=197), Fighting Axe (http://c-rpg.net/index.php?page=equipshop&cat=onehanded#!?page=itemdetail&id=197), Langes Messer (http://c-rpg.net/index.php?page=equipshop&cat=onehanded#!?page=itemdetail&id=3298)
^ Also, not to confuse. The sickle, fighting axe and langes messer are all one handed weapons with a two handed alternative mode.
I personally never used one-handed weapons as a Pikeman because I felt I had better choices in other single slot 2h/polearms. But you can try any and all of these out, fairly inexpensive and amazing sidearms.
EDIT: I totally ignored him asking about pike vs longspear. Sorry about that! I myself have used a MW Pike and MW Longspear, both are amazing weapons. There are a few key differences to keep in mind, the Longspear is shorter but makes up for it by being faster and stronger. The pike is the ultimate support weapon that takes practice, that length will strike terror into your opponents. One thing that is incredibly important to know when using a Pike is that when somebody blocks your attack, your speed delay is so slow that you can't block that same person incoming in time. You will be forced to draw a sidearm in that case, whereas the longspear has ample time to pull their block up after being deflected.