Philosophy: Models can be different:

(1) simple formula, no Add-In
(2) named formula
(3) array formula
(4) Add-In function
(5) VBA (Basic) function, user defined
(6) Excel4Makro function, user defined
(7) VBA (Basic) procedure (Sub)
(8) Excel4Makro procedure
(9) .DLL or other external modules

All of the above can be mixed at model or formula level.

Comparison:

The following is a negotiable table without any reference, since it could be subject to hard fights between evangelists of the 9 methods above. At least, I did not include any weighing :-)

                          poor..... .... ok....... .... fine.....
ability .............     .........      1234.....      ....56789
elegance of design ..     1........      .........      .23456789
flexibility .........     .........      1234....9      ....5678.
comprehensibility ...     ..3......      12.......      ...456789
length ..............     1........      ..3......      .2.456789
performance .........     ..3......      12.456...      ......789
need of memory ......     12.......      ..3......      ...456789      
language portability      ...4.....      .........      123.56789
program portability .     .2345.7..      .....6.8.      1.......9
external dependency .     ...4....9      .........      123.5678.
security (viruses) ..     ...45.7.9      .....6.8.      123......
safety of property ..     12..5678.      ..34.....      ........9
expense of making ...     ....56789      ..3......      12.4.....

Language portability means multilingual use of one platform.
Program portability means cross platform (f.e. Excel and Open Office) use.

Compatibility:

+-----------------------------------------------------+
|Excel:                                               |
|Named formulas with or w/o Excel4 Macro Functions    |
|Excel-Add-Ins                                        |
+---------------------------+-------------------------+
|Excel, Open Office:        |Excel, SpreadCE:         |
|Array Formulas             |Excel4 Macros (Fct.+Prg) |
|VBA oder anderes Basic     |                         |
+---------------------------+-------------------------+
|Excel, Open Office, SpreadCE, Lotus:                 |
|Simple Formulas                                      |
|Usage of .DLL or .XLL                                |
+-----------------------------------------------------+

In this overview you see exclusivities of program combinations, viewed from Excel. Lotus Macros or procedures are not commented.

Even simple formulas surprise: 5011 does not work under Open Office. SpreadCE works w/o problems (for NU=0).