Here are some raw numbers. This is their offense based on their cards in one specific park. (Not a home/away split like coffeeholic worked out some nicely.) I did Shea, and one neutral park (Montreal). All numbers are out of 108 dice chances.
VS. LEFTY AT MONTREAL (Single=1-10, HR=1-10)
.....................Belliard..........Boone
Hit....................35.0.............22.6
On-Base............49.0.............41.6
Total Bases........59.7.............51.5
VS. RIGHTY AT MONTREAL (Single=1-10, HR=1-10)
.....................Belliard..........Boone
Hit....................24.9.............24.9
On-Base............35.2.............33.9
Total Bases........41.0*............41.8
(* Belliard is W vs righty)
VS. LEFTY AT SHEA STADIUM (Single=1-6, HR=1-3)
.....................Belliard..........Boone
Hit....................32.6.............19.15
On-Base............46.6.............38.15
Total Bases........53.1.............40.7
VS. RIGHTY AT SHEA STADIUM (Single=1-6, HR=1-3)
.....................Belliard..........Boone
Hit....................24.2.............22.5
On-Base............34.2.............31.5
Total Bases........40.0*............35.2
(* Belliard is W vs righty)
So in a neutral stadium, Belliard is a slightly better hitter. In a pitcher's stadium, the difference is greater. As PBTR pointed out, Boone's superior fielding range also has to be considered.
Bottom line to me is it that it is reasonable that they are similar priced. In a hitter's park, I'd lean towards Boone; in a pitcher's park, I'd lean towards Belliard.