The current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) reads as follows:
P-220.127.116.11 Systems composed of rings and chains (exclusive of linear phanes)
Two methods are recognized to name systems composed of rings and chains
(exclusive of linear phanes).
(1) Within the same class, a ring or ring system has seniority over a chain. When a ring and a chain contain the same senior element, the ring is chosen as parent. Rings and chains are chosen regardless of their degree of hydrogenation. As a consequence, this approach prefers the choice of a ring over a chain in systems composed of cyclic and acyclic hydrocarbons.
(2) The context may favor the ring or the chain, so that, for example, substituents may be treated alike or an unsaturated acyclic structure may be recognized, or the one chosen has the greater number of skeletal atoms in the ring or in the principal chain of the acyclic structure.
So far, we have three possible names:
Method (1): “(3-cyclopentylpropyl)cyclohexane” or “(3-cyclohexylpropyl)cyclopentane” (ring preferred to chain)
Method (2): “1-cyclohexyl-3-cyclopentylpropane” (chain preferred to ring)
(…) For selection of a preferred IUPAC name, see P-52.2.8.
P-52.2.8 Selection between a ring and a chain as parent hydride
Within the same heteroatom class and for the same number of characteristic groups cited as the principal characteristic group, a ring is always selected as the parent hydride to construct a preferred IUPAC name. In general nomenclature, a ring or a chain can be the parent hydride (see P-18.104.22.168).
This leaves us with two possible preferred IUPAC names:
“(3-cyclopentylpropyl)cyclohexane” or “(3-cyclohexylpropyl)cyclopentane”
P-44.1.3 Seniority order only for rings and ring systems. Criteria that apply only when the choice for parent structure is between two or more rings or ring systems are given in P-44.2.
P-44.2 SENIORITY ORDER ONLY FOR RINGS AND RING SYSTEMS
P-44.2.1 If application of P-44.1 does not effect a choice, general criteria for determining ring seniority given below are applied successively until there are no alternatives remaining. (…)
The senior ring or ring system:
(e) has the greater number of skeletal atoms;
Therefore, the preferred IUPAC name is “(3-cyclopentylpropyl)cyclohexane” (not “(3-cyclohexylpropyl)cyclopentane”) since cyclohexane has more skeletal atoms than cyclopentane.