1) Really no impact on the situation. Both utilities are for windows.
2) XP is icky
Upgrade to 98.
3) WinRar saves you 20% on the imagemod. 30% on Gritecon.
For my Starfury P&N-ism mod, WinRAR compresses ALL THREE campaigns smaller than zip can compress one campaign.
For anybody on Dialup, it is well worth the download.
4) With winRar, you have the same functionality.
Rightclick, "add-to XYZ.rar"
PS:
Also, it is worth noting that winRAR can make zips smaller than winzip can. Those extrasmall zips are still fully readable by winzip.
The zip values used above are winrar's extrasmall zips.